It's bats. I ended up chasing one out of the house last night. I strongly suspect there are more, though all's quiet this morning.

It had been very cold for a few days and got very mild last night, so I think their hibernating was a little disturbed. Will figure it out when me mate gets home.

I daren't light a fire in it - it's propane and likely the fumes would leave the critters unconscious and blocking all manner of things guaranteed to start a fire or send propane where I don't want it.

Fortunately, I ain't a-scairt of bats.

I'm real scared of propane, though.