By "key" I assume we are talking about something that fits in a lock and not 2.2 pounds of a white powder?
If your employer was managing this, they would have searched you while you were still in their building and found the key then. Unless your former employer is some three letter federal agency, I agree with the other answers. Que Bono? Who benefits? There are smart employers and stupid employers but none of them act like they are in a bad movie. Unless there is something you are not mentioning, what would their motivation be? Even really stupid employers have motivations. Generally laziness.

