If this were a mystery story, the prime suspects would be the people who helped you search for the key :)

If you have a spare computer and some spare time, you could surreptitiously set up a few sneaky cameras. Just don't tell those people. If you think this is supernatural with, say, p>10%, this sounds like a fun thing to investigate. Well, fun now that you're out of a house that was for one reason or another correlated with you having night terrors.