@evan yay!! this reminds me of the wheel lock-nut that got lost in the same place in our car last year. It wasn't until *after* we had the tire shop break the locks on the wheels and replaced them with standard nuts that I found the lock nut when I pulled the mat out from under the driver's seat and the glint of the metal nut caught my eye ;)
I ran into the house, grabbed the car keys, and went out for a last pass. It wasn't anywhere. Then, I reached down between the seat and the centre console, and I could feel it with my fingers! I pulled it out; my passport was there! It must have slipped out of my pocket on the ride home.