Wev’e had 2 MAHOOSIVE bumblers in our bathroom lately (safely evicted out the window), they can only have come in via an airbrick behind the cupboard.
Quite possibly, they look for small dark spaces to build their nests, which are pretty small.
I have an old shed in the garden, and the bottom of the door is broken away on one side. Every year, bumblebees nest somewhere in the shed, and you can sit and watch them flying in and out through the hole at the bottom.
If I open the door to get tools out, they get really agitated, because they haven’t got the small dark target to fly towards, it’s just one big black space.
I now try to keep the door closed as much as possible to avoid upsetting them.
What with the bumblebees and keeping the hedgehogs fed and watered, along with the local birds, I sometimes wonder if I’d be better off with just a cat!