Boaz was doing something similar. He'd run up to the top of the stairs and make a little puddle up there, like he was marking. But being so young, I wondered about it. He did it a few times in the exact same spot, and I recently had the carpet cleaned thoroughly up there. He stopped peeing inside a long time ago, thankfully. I think my little guy was just a little spunky. He's very good at running outside, but just a few times put his mark on that one spot.