I have a colleague whom I don't know particularly well (have been on a few nights out but never really chatted to him much one-to-one), he occasionally comes through to our department for a chat since we're by the coffee machine at the door.
In the 4 months i've been at this desk, there have been several examples of rather severe moaning about conditions/pay/holiday entitlement instigated by this employee, loudly and vocally, across the whole team. There has also been 2 occasions where the employee collared me at the coffee machine and engaged in such talk (once when I asked him if he had a client meeting in our conference room as he was in a suit, to which he replied no i'm going for an interview at 2pm... the 2nd time where i bumped into him on the way out the door, he was leaving early (he starts at 9am, i start at 8am) to attend another job interview).
AIBU to not want to hear any of this, and to think it's just - well - inappropriate? We're at the same level, so it's not like he's complaining to a boss. But being so blatently unhappy about working conditions here is something I just dont' want to be party to!
I originally suspected he might be a mole (paranoid much?) because he's just a colleague, not a friend, and maybe this was something the new project manager did to assess motivation levels in staff? But now I think i was just being paranoid 
Every time this happens at our bank of desks, i don't join in the conversation unless addressed directly, and if talking to him personally, i make non-commital noises but don't agree with anything he says in case I end up getting labelled a troublemaker too!
Example: this morning he was in here complaining about how his wife gets an extra bank holiday for the royal wedding, but we don't, and how "stingey" our boss is, how much he's looking forward to going to an interview at 4.30pm today... etc.
AIBU to want to scream "keep it to yourself"? And hope the guy finds another job to moan about soon?