He was trained and we had no problems until recently. He also just started lifting his leg when he urinates. He has access to the garage through a doggie door. He has free roam of the yard during the day but continues to mark the garage. It is very difficult to catch him in the act. Any ideas??

First question, has he been neutered yet?

No he has not been neutered. I've read different recommendations for neutering dobes at different ages with concerns about doing it too early could affect their growth.