Even though you said there's no evidence, I'd still lean towards a critter of some sort. We get some field mice coming in when the weather starts getting colder and our cat will just camp out next to the dishwasher and stare at it, I'm sure she hears them in the wall behind it. We never see or hear them, every now and then we'll see some pelletized evidence if we're cleaning or moving things around.
Is he seeing a reflection off of something on the wall in that spot?
Or he could just be loopy.
