The Secret Panel (Hardy Boys, Bk. 25)

Another exciting mystery begins for Frank and Joe Hardy when they help a stranger who has had an accident with his car. The man introduces himself as John Mead, owner of a nearby estate. After he continues on his way, Frank finds an odd-looking house key which belongs to Mead. But when the Hardys try to return it, they learn that John Mead died five years ago!