Mystery Solved

This little culprit was making a 'lump lump lump' sound whenever the car moved. The car is big. The screw is small. This is so very amazing to me.

John at the dealership brought it to me while I waited. We were also having the radio replaced (Thank God for warranties), otherwise I probably wouldn't have had it looked at, just continued my puzzlement. He said they found it in one of the rear tires, and that the 'lump' sound was the screw against the pavement every time the wheel went around. The head of the screw took some serious damage, too!

The *really* good news? It punctured only the tread at an oblique angle--no hole, no leak!