Hi Gillian,
This defo sounds like a mouse...but with all those cats you are not very likely to see it ha ha. I had one in my last house which seemed to be in the skirting boards. My cats used to sit for hours looking at the skirting boards but nothing ever appeared..thank goodness because I wouldn't have wanted them to kill it.
Eventually, it went away...or worse..died. I hope it didn't die. My thinking is that if it managed to find its way in it must be able to find its way out.
Last winter, while I was moving some gear into my new house, long before I moved in, I found the most exquisite little mouse nest inside the garage. I had stored some boxes and things there so it wasn't empty but I was just preparing to sweep the floor on this particular day...and as I moved one of the boxes aside, I came across a little mound of dried grass and shredded paper....and a few other materials from my garage that the little creature had thought to include in her nest. It was sooooo beautiful. The little mouse wasn't in it...and I would think was preparing to have babies, but I just left it where it was...It wasn't hurting anything.
It was some time before I actually moved into the house, by which time everything had gone. I hope, by finding it, I didn't frighten the little creature into abandoning its home....bless.
I think it was just a little field mouse or something...coming in from the cold.
Best wishes,
Roz