I think they can see them--and sense them. A good friend of mine used to own a Metaphysical shop that was in an old building that had all kinds of wierd energy. When I brought my normally mellow Howard to visit her one day, his eyes got HUGE and he was just looking all over as if he were seeing things that weren't there. Then a couple years later, I moved to an apartment that is actually an old house that has been divided up. Ever since, Luna has been obsessed with my closet door, which I always keep closed just to shut out the clutter inside (it's a fairly large walk-in closet). She will sit by that door and meow and meow and sniff at it as if there's something in there that she wants or wants to see or is talking to. She doesn't behave this way with any other closed door in the place, not the door to outside, not the bathroom door (which, too, I usually keep shut)....not even the door that leads to the cellar. Just the closet door. Sometimes Howard will join her in her obsession.

Not that I think The Boogyman lives in my closet (that's too cheezy), but I do think that there is some sort of energy in that part of the apartment that attracts Luna's attention and makes her act like that.