The tall, dark-haired figure flitting through the lobby with a lissom blonde on his arm may have looked distinctly familiar, but the discreet staff at Cardiff's premier five-star hotel were never going to let on.
Not that they were being rude - employees of St David's Hotel and Spa are renowned for their polite attentiveness. When it comes to a valued customer by the name of David Tennant, however, they are aware that it really doesn't do to pry.
After all, who is to say how long tonight's companion will be on the scene? As became apparent this week, the Dr Who star is romancing perky blonde co-star Georgia Moffett - his fourth known conquest from the programme's cast and crew since he landed the role three years ago.
And this, it can be revealed, may just be the tip of the iceberg. A source on the hit BBC programme says there have been 'at least five and probably more - but he's just so darned charming everybody still loves him to bits'.
'David may not have the looks of Brad Pitt, but he is an absolute genius when it comes to impressing women, and he's having the time of his life,' says the insider.
'He spends a lot of time filming the show in Wales and let's just say there has been a lot of traffic through his apartment in Cardiff Bay. He takes women to St David's Hotel because it's quiet, luxurious and the staff love him there.
'But it has become a standing joke that you never know who David will walk through the door with next. Some of the staff have been having bets on whether it will be a blonde or a brunette.
'But the most extraordinary thing is he manages to stay on good terms with everyone. Maybe it's his Scottish charm or the fact he is a decent man, but somehow he manages to square the circle and keep everyone happy.'
The 37-year-old has certainly kept himself busy while filming Dr Who at a massive BBC studio on an industrial estate at Treforest in the Welsh valleys.
First, he dated Sophia Myles, who played opposite him as the extravagantly titled Madame de Pompadour. She admitted to being utterly smitten - she used to carry a Dr Who doll in her handbag during the two-year relationship - and was devastated when he left her in October for Bethan Britton, who works in the BBC Wales contracts office.
A clearly smitten Miss Britton confirmed earlier this year that she was in 'a serious relationship' with Tennant, but within weeks he had moved on to another member of the Dr Who crew, second assistant director Jennie Fava, who he escorted on a series of dates.
This proved to be another fleeting romance, however, and this week it emerged he has been spending nights with Georgia Moffett, 23, who played his daughter in a Dr Who episode. She is also the real-life daughter of Peter Davison, who played the Time Lord between 1982 and 1984.
As one admiring male crew member puts it: 'Where does he find the time - let alone the energy?'
The BBC insider told me: 'To see David in action with people in general and with women in particular is a work of art in motion. He is such a charmer that he can get away with anything.
'He always has time for everyone and the women love that. If there are any children who come to filming, David always makes a fuss of them and poses for family pictures.
'The women love him on the set, he could have his pick of most of the single girls - and probably a few of the married ones, too, if he wanted.
'It's a bit sickening for the blokes, though. You would love to hate anyone that good with women - but even the men have to like him as well. There's a few gay guys around and they love him just as much. But they reluctantly accept he's a devout heterosexual - and practising very hard at it from what I can see.
'He's been in the gossip columns about going out with a few girls on the set, but, believe me, that is not the whole story. I know he's been out with at least another two girls. It may have just been for a late-night coffee - they're too discreet to kiss and tell - but they were very happy about it.'
Another member of the Dr Who team insists that Tennant's roving eye is down to the high intensity on set, where working days regularly run from dawn until dusk.
Friends who have known him for longer, however, suggest there may be another, altogether more deep-rooted explanation: his upbringing.
As a son of the manse - his father, Sandy, is a Presbyterian minister who was Moderator of the Church of Scotland for some years - he makes an unlikely Casanova. (A part he played to great acclaim in a BBC drama of the same name in 2005).
As a slim, gawky teenager, he was a regular churchgoer in his home town of Paisley, near Glasgow, where his father's charismatic sermons inspired him to pursue a career as an actor.
This background, coupled with the fact his parents were happily wed for 45 years until his mother's death from cancer last year, has left him with a lasting reverence for the institution of marriage.
'David is regarded as something of a lothario these days, but there is nothing he would like more than to fall madly in love and settle down to a lifetime of monogamy,' says a friend who has known him since his days at the Royal Scottish Academy of Music and Drama.
'The problem is that his romantic ideal is so hard to attain. His parents were an amazingly dedicated couple and he idolised them.
'So far he hasn't met the right girl - but in this day and age the notion of being married for the best part of half a century is a bit unrealistic.
'He needs to learn to compromise, perhaps ease off on the work so he can get his personal life in order. But that's not David. His expectations in life are ridiculously high, which is probably why he's so successful.'
Intriguingly for someone so ostentatiously successful with the opposite sex, during the early years of his career tongues wagged about Tennant's sexuality - partly fuelled by his relaxed attitude to the rumours. When he was at drama school, several of his contemporaries believed erroneously that he was gay.
And an interview with his friend and writer Arabella Weir was headlined: 'It's OK to think the Doctor is gay.'
Friends say the 37-year-old is unconcerned about the rumours. One suggested they were a handy way of damping down speculation about the amount of time he spends with attractive young ladies. One female crew member on Dr Who, who asked not to be named, said gossip about his sexuality was 'hilarious'.
'It's hard to put into words how charming he is,' gushed the twentysomething blonde. 'David has huge brown eyes and when he turns the full glare on you then no girl with a pulse is going to get away from him. He may look stringy and geeky, but that man has more sex appeal than anyone I've ever met. He's the nicest man in showbusiness.'
This, it must be said, is not an isolated opinion. Tales of the Doctor Who star's largesse abound and he has a deserved reputation as a star who has not lost the common touch.
On one occasion, when recording a voiceover for a documentary, he was approached while having lunch by a minion who worked with disadvantaged children in his spare time.
When the man explained that his charges were big fans of Doctor Who, Tennant spent the entire break recording an improvised skit in character as the Time Lord on a mobile phone camera.
Friends say it is this spontaneous generosity of spirit (he neither sought nor received publicity for the gesture), coupled with unshakeable self-belief, which makes him so irresistible to women.
In Georgia Moffett, however, some suggest he may have taken on more than he bargained for. Unlike Tennant, her parents - actors Sandra Dickinson and Peter Davison - had complicated relationships.
Dickinson and Davison were married to other people when they met, and by the time their marriage dissolved after 14 years, it had descended into a series of bitter rows. They are barely on speaking terms and friends say this, coupled with the startling fact that at 23 Miss Moffett has a six-year-old son, has left its mark on her.
How long her romance with Tennant will endure remains to be seen. Last weekend they were photographed emerging from his modest apartment in Crouch End, North London, after spending the night together, and the following evening stayed over at her home in West London.
Since then, however, both have been keeping a low profile. Tennant is in Stratford-upon-Avon, rehearsing for his stint in the title role of the forthcoming Royal Shakespeare Company production of Hamlet.
The only sign of Miss Moffett, meanwhile, was a boy's bicycle propped up against the wall of her father's £1 million house in Richmond. A smiling Peter Davison declined to comment on his daughter's relationship, simply stating: 'You're not going to find her here.'
A mutual friend says: 'It's early days for David and Georgia and I'm not sure if it will go anywhere. He's almost 15 years older than her and besides their profession they don't have a huge amount in common.'
Even some of Tennant's admirers admit he has a steely side. One TV interviewer who has known him for years says that since landing the Doctor Who role, he has developed the celebrity tic of referring to himself in the third person.
'There is an egotism and ruthlessness about him. The most important person in David Tennant's life is himself, which means he isn't ideal boyfriend material. So long as it's convenient he'll make a woman feel as if she's the only girl in the world. But career comes first.'
As Arabella Weir attests, 'steeliness is his defining characteristic', and many believe it is this unwavering commitment to his career which has rendered him incapable of sustaining long-term relationships.
Those who have known him for longer, however, point to the impossibly high standards set by his parents' wonderful 45-year union.
'He still firmly believes that one day he'll meet someone special and they'll be together for ever,' says an old college friend. 'But that's not going to happen if he changes girlfriends as often as his socks.'