While out exploring the recently opened Massport Hanscom trails, I came across one of these stone markers, and immediately knew that a geocache had to be placed nearby.
You are looking for a small Lock & Lock near a stone marker that has a B on one side and C on the other! I'm guessing the marker marks a corner point on the Bedford / Concord town line. The cache is hidden quite near this marker, in such a way that I don't even know which town the cache is actually in, so I'll just say it is in Bedcord. Or is it Conford?
There is a DD gift card in the cache waiting for the FTF.