doesn't really sound like either arrogance or lack of respect. He didn't mention it either because he forgot or because he didn't want you to know. if by mutual girlfriend you mean she was actually his gf at some stage, you're right to raise your eyebrows. Anyway, why not just tell him 'I saw X today and he told me that Y was with you in London, how come you didn't mention her?' Try not to sound jealous about it (though I suspect you are a bit, as I would be), the point is to observe his reaction... shifty or not?