Why is our cat peeing everywhere?

My roommate and I recently took in a friend’s cat (temporarily) due to a bad situation the friend was in. For the first few weeks everything was fine. The cat seemed to be adjusting well and got along with both of us.

This weekend, I spent one night away from home (roommate was still here) and when I got back, the cat had peed on my bed twice. The next day, he peed on the couch, and today, he peed on my roommate’s floor.

We keep his litter box fresh and clean, and he clearly understands its purpose since he used it appropriately for the first few weeks he was here. What else could be going on with him? How can we stop him from stinking up all our stuff?