My Belle does the same thing, has since she was pup. She's a little rascal about it. She'll scrape at the cushion or blanket or whatever soft thing and wee on it. This happens very infrequently, maybe once every couple of months. I think she is marking with her scent, because sometimes she'll roll around in it (or lick it). We have had her checked by the vet as well, all plumbing, etc working properly. She seems to pee on things that are freshly washed as well. She's a perfect little girl otherwise, just has this peeing problem. Hope you can work yours out! We just keep an eye on her and she sleeps on a waterproof bed.
Desrae, guardian of two tri-coloured cavaliers, Belle 3 years old and Bobby 1 year/8 months.