Help with my dog

Hey pet lovers!

I hope there are some animal experts here that can help me with this issue I'm having with my dog. He is a Pomeranian, about 12 pounds and he just turned 8. Recently he has started peeing on the wall in a hallway of my house. I can't for the life of me figure out why this is happening. I haven't be able to catch him at it. I just smell it after the fact and clean it up really well. We do have a daughter that is two that he had a hard time with when she was first born. Nothing else has changed in his surroundings though and obviously her birth was a while ago. As was her learning to walk. Any ideas for me? He doesn't do it that often (like maybe twice a month) so I haven't been able to see him do it.