Heh. There -was- a semi-hidden door in the "alien geometries" room going outside, actually, although it had been boarded up from the other side, and I've since replaced it with well-reinforced masonry. And the original door was spray-painted in red with bizarre symbols. (I'm not making that up. Seriously, imagine this: First, there's a room that doesn't seem to join together properly. Now there's this apparent wooden wall, in a room mostly made of cinderblock walls, covered in weird symbols in red spray paint. The wall swings open if you can get a fingertip grip on the edges of the wood - into a featureless wooden wall behind it. -Nobody- liked going into that room.)

The only way a key retrieval would have been possible is if the sneaky person had reached out from a heating duct and grabbed it. Which is conceivably possible - the ductwork is loose and could be pulled down from the basement.

...considering my girlfriend refused to be in the house for more than a month after I showed her a haiku inspired by the noises inside the walls as being rats with broken necks being thrown down from the attic by a creature living there (the house was -awesome- writing inspiration), I don't think I'll relate that particular explanation.