Since you already know that it was a human who threw the key under the basement door, you already know that it was "thown", as opposed to "slipped" (Not that it would be any difference anyway).
And to answer your question, you also know that As far as the Howard (the owner) knew, there was no person in the hotel besides those mentioned in the story. Those mentioned in the story are you, Lucy, Alicia, Bill, Charlie and of course Howard himself.