Is he gay?
Goes on holiday with female who's not his girlfriend - check
Leaves a nightclub with two men who he doesn't know - check
Leaves an apartment on his own after staying there for a short time, presumably the two lads he went with have left to go elsewhere, have they dumped him after "using" him - check
Seems to have a few female friends who he's not involved in relationships with - check
The 2 blokes he left the nightclub with have to my understanding disappeared, surely it can't be difficult to trace these 2 if they booked the house on Air BnB?
Something very odd obviously, still the possibility of him being alive and it's a scam that's gone wrong, if he is alive he's got a lot of questions to answer.