6yr Potty Trained Chihuahua Urinating in House

Hi, I have a neutered 6yr old potty trained Chihuahua that started peeing in our guest bath in Sept. A trip to the vet revealed he had a mild urinary infection. Put him on meds and that seemed to stop the problem. It started again in November. Another trip to the vet, along with some blood tests showed he is perfectly healthy. I started to crate him again at night. Sometimes he will pee in the guest bathroom that has been cleaned, on/off again during the day when we are gone. There is a pet door that is always open during the day that he and our cat use. The dog & cat have grown up together. Not sure why he is doing this other than he is just too lazy to go out? I’ve thought about spraying some pet No No type stuff where he pees, but afraid he will find another place to do the job and not sure if it works. Thankful that he has chosen the bathroom, but still… why? Other then retraining him by crating at night, do you have any suggestions?