My parent's cat, Casha, whom we got when I was about 7, still does that. She's getting on in years now, obviously, but she still carries around balled up sockes and slippers. She does it at night, usually in the wee hours of the morning, and moans pitifully when she does it. But she'll knock it off if you take the sock away. Anyway, the vet said it was a hunting behaviour, but I think it's a maternal thing. She never had her own kittens. Plus, I lived there last summer, and I got Sammy. He was 10 wks old when he came home and we stayed there for a couple months, until I went back to school. And Casha didn't do the sock/slipper thing AT ALL while Sammy was living there. Seems persuasive to me.