A cat may pee in its bed to identify the bed as the peeing cat.s bed. The cat may pee in its bed because of a urinary tract infection, a cat may pee in its bed due to hormonal changes. A cat may pee in its bed because it doesn’t know better.

Is Spots male or female, and are you positive that it's Spots that's peeing in the bed?

It's unusual for a cat to soil where it sleeps, but not impossible, so I would be suspicious of another cat/animal scaring Spots off and then marking the bed. Any chance you have a camera you could set up?

We once had a cat house set up for a stray and discovered she was being scared off every night by raccoons. She ended up living in our garage instead.

As Jim mentioned, Spots could have a urinary problem so a trip to the vet wouldn't hurt. Otherwise, try placing a litter box near the carrier and see if he/she uses that instead. Are the cats old or arthritic?

If the carrier is plastic, I would wrap it in some towels to help insulate it at night since plastic doesn't hold heat well. Also, if you have any wild animals in the area....raccoons, opossums, etc., Spots may appreciate the carrier elevated (on some type of table) so he has more time to escape invaders.