Earlier Popsie got a bug up his butt and went running up the stairs to the main part of the house. I was watching him go and he stopped inches from the corner of the wall. Then he sat staring at it like "Hey where did that wall come from."
At least he stopped.. when Niko is chasing the invisible mousies, he runs into things all the time. I think he believes he can magically walk/run through walls. When things go thump in the night, it's just him being a li'l nut.