Nailed It!

The rear passenger-side tire on my vehicle has been losing air.  At first, it just needed to be aired up every other week, when the 'low pressure' alarm sounded.  Yesterday, I got the alarm, so I aired it up before going to bed.  

This morning, it was low, again.  Before leaving the house, it was re-pressurized.  By the end of the day, it lost over half of its pressure.

Upon returning home, the tire was removed.  This is what I found:

One of the culprits, once pulled from the tire with a pair of needle nose pliers: