I think there's a wasps' nest in my chimney.
Heard a buzzing yesterday morning, and found a wasp getting irritated with the net curtains - so I let it out, while wondering where it had come from, as there weren't any windows open. Then a bit later, there was another one. Strange, thinks I, but I'd had the back door open by then, so didn't really think any more of it. Then later again, I heard more buzzing - couldn't see anything in the window, then saw it down by the bottom of the gas fire (disconnected, due to the unique way my landlord is funded motivated). So I let that one out too. And then the fourth one, buzzing at the other window. And I could hear angry buzzing for a while behind the fire.
Now either it's one wasp doing laps for a bet, or the buggers are in the chimney. And I really have no idea how you'd get to them. Cock.