I'm pretty sure there's something in my house, as well. And the way I can tell is through my cat.
At night, when I get up to use the bathroom, the cat will follow me (he likes to drink from the tap, for some reason). On more than one occassion, when heading back to bed (again, the cat following), he'll pause in his step and peer into the living room - not moving a muscle. Always takes a couple of nudges of my foot or saying his name to snap him out of his rigid stance.
Freaks me out - as there's no visible reason he would do that. Though it unnerves me when he does it, I shake those feelings away, scold him for making me nervous and head off to bed.
There are other points where the cat seems to communicate with something unseen; either mewing at thin air or chasing something that can't be seen.
Last night was a little odd, though - I felt a weight sitting by the foot of the bed - and thought nothing of it; thinking it was my cat. Only, not a few moments after feeling that gentle weight, there's a heavier weight and the sound of claws digging into the sheets (this would be my cat jumping unto the bed - he's a heavy boy), and he's come to settle by the lighter weight.
This didn't bother me all too much - I felt no negativity with that lighter weight. But it's nice to know my cat is a protective baby.
smile