Someone may already have said this:

For some people seated in the back of the plane, having their carryons in a forward bin is desirable, because they don't have to heft it up the aisle at the end of the flight. This is only problem if 1. you needed something in the bag midflight or 2. you didn't see it get moved.

I would have buzzed the FA and said, "I believe that's my bag you just moved," and then ask that she return it to its original spot after takeoff so I could get to it OR that they make sure it's waiting for me by the cockpit when I get off the plane because I didn't see where it went.