I decided to have a look for its source this evening.
It seemed to be coming from somewhere near the door to the lounge, so I began moving things around. First everything in the hall. No sign of a source there, which meant it wasn't anything to do with the building work. My heart sank. I now had a pretty good idea of what I was looking for, and a couple of places where it might be.
Nothing showed up when I moved the sofa. Perhaps I was wrong.
Back to the lounge door. A pile of comic boxes was next to be moved. I'd reached the bottom, when I saw it.
I was right.
A dead rodent of some description, probably a young rat. Gently decaying on top of a couple of books at the bottom of the bookcase.
One night, probably a week or so ago, the cats brought in a little friend for, err, games and dinner. However the games were a little too succesful, and dinner was postponed. Their little friend must have crawled behind the boxes and into the bookcase, and then expired.
It's now far away, wrapped in several plastic bags (unfortunately along with the books). The bookcase has been soundly disinfected, and the carpet thoroughly cleaned. There's also plenty of deoderant powder on the floor.
I think I need a big bar of chocolate.
And next time I wake up in the middle of the night, and wonder if the cats are chasing mice in the hall when something goes "squeak", someone remind me to go check, not roll over and go back to sleep. Things will be so much more pleasant in the long run...