Here's a trick I've used twice already. Before you start putting your lower together empty your bagless vacuum. If you lose something vacuum the room/area it went to. It should be in your dirt container. You can do this without emptying it first too but it's not nearly as pleasant. Found the same detent twice using this method.
That'd work... except I have a bagged vacuum cleaner, and I had NO idea which way the part went. Also, there's heavy furniture in my gun room, and you know the rule: the heavier the furniture, the more likely the flying part will go behind it