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.

Nope. I'm just going to defy you here, since this sort of restatement is occasionally bad. Four different people searched for this key (that you know of >:D ), and I doubt you said to every one of them "don't bother, I searched well enough to exclude me having overlooked it" to each one.