Any question answered...

So far I've half pulled the accessible one off the ceiling trying to work out how to open it (I think a screwdriver is the way to go). I followed that up with climbing a ladder to the one I thought was beeping but, putting my ear next to it, that doesn't seem to be the case.

I'm now pondering whether it is something buried in the loft or possibly next door but it will be another night of trying to ignore the chirps every 38 seconds.