No, you're correct. I was going to point this out before I saw your comment. The example I had in mind was the Rabbit's Foot in Mission:Impossible III. Perfect

No, you're correct. I was going to point this out before I saw your comment. The example I had in mind was the Rabbit's Foot in Mission:Impossible III. Perfect example of McGuffin: it motivates the entire plot of the movie, but at no point it becomes clear what it is (other than Simon Pegg speculating about it). It's in a canister, the bad guys want it, and Tom Cruise has to stop them from getting it.