Neutered Dog Spraying on Couch?

I have a 10 year old, neutered Lhasa Apso, indoor dog. He has no known health issues. He knows to go out and will ask and be rewarder when he goes outside.

The problem started over the last 2 years. The 1st year I blamed on a neutered cat who has now passed. The dog will walk up to the end of my couch where it touches the floor, a fan sitting on floor, the leg of a wood chair, the bottom stair post, etc, lift his leg and spray those and other items casually. I rarely see him do it, but a few times he had done in front of me, smiles, and acts as though he had just scratched himself.

He is taken out regularly and rewarded when he goes. He will let us know when he does need to go out, so he understands what to do. He will wake me up each night about 4 to 6 AM to go out. This behavior has only started about 2 or 3 years ago. I had been blaming one neutered cat who has now passed away, so I learned that it was not him. Once in a great while, he will do it right in front of me with a nonchalant look and then rub up against me as though what he did was OK. Help. How do I convince him not to mark my household items?