I don’t mind the occasional 3rd wheel and I get along with DP’s friends... but recently when I’m at home with the DC (and if DP doesn’t have work the next day) my DP goes to his best friend’s girlfriends house and stays there overnight. With his best friend there as well of course, but noone else.
I don’t particularly think there’s anything shady going on I just think it’s weird
He point blank refuses to use taxis. He drives up to her house, drinks with them and leaves his car on the drive, sleeping there overnight until the next afternoon when he can drive again
If it was me I would get a taxi up and then a taxi home again. It’s not like he can’t afford it 
My DC aren’t his, so even if he stayed here he wouldn’t be looking after them (so he hasn’t abandoned any responsibility).
If im free for the weekend he doesn’t go, probably because he knows I wouldn’t want to sit and drink in some else’s living room. AIBU to think that he’s intruding on their privacy a bit 
His best friend doesn’t live there by the way, this is a new relationship of only a few weeks