I'm beginning to suspect we have bats in the house.
Got home from an evening out last night to find one zooming around the new extension area.
That's the second time in a few weeks.
The extenesion area has big doors which I used to let it out, but they had not been open yesterday due to the heavy rain, so it didn't come in that way.
I'm suspecting they are in a roof void-space, because these is some concealed lighting on high up shelf type things in the extension area, and inspection reveals several openings for wires etc into the roof void-space. I think they have gotten into the living space through these openings. It's the only possible way in I can see.
I will probably have to block up access into the living space, and leave them be in the void space.