The night that we had brought home our two kittens, now adults, I was chopping up a carrot while making dinner and I heard a sound in the kitchen cupboard. Our kitchen was built by an idiot so the two doors in the corner cupboard have a gap of a couple of inches where they should meet and don't. So I went over to the cupboard and leaned down to listen for the noise again, and a cute rat face poked its head out of the gap with a look on his face which said, "A carrot! Is that for me?"
I found out that some kid-monsters across the road had let him escape and he may have made his way from half a block down the road into our house. Clever!
I couldn't keep him, because the combined smell of his cage and him affected hubby's asthma, but I would have kept him. The kittens would have coped.
Someone from around the corner took him but he was dead, despite his luxurious cage with upstairs downstairs and built in toilet area, because the woman refused to believe me about not using clay based cat litter because it causes sudden & massive respiratory problems. He got sick & died within a day.