Dog Started Peeing on His Dog Beds?

My husband and I adopted a 4-5 year old Dachshund about a year ago. We believe there may have been a history of abuse. Recently, he has started peeing on his dog beds. It first happened about 8 days ago, and again about 3 days ago, and then a third time this evening. All three times I have been in the room and caught him doing it. He is taken outside frequently and seems to be having no issues using the bathroom. What could this be from?

We don't have a ton of money right now and I am nervous that if I take him to the vet they will charge me an expensive check up fee and say he's perfectly healthy.