I noticed, in an article in the Wall Street Journal this morning, a note regarding one of the balloons that had recently been shot down: 

A possible explanation for one object has emerged online. A civilian balloon club, the Northern Illinois Bottlecap Balloon Brigade, said on its website that one of its amateur weather balloons, Pico Balloon K9YO, “was missing in action” shortly after flying at 38,910 feet near Alaska.

Of course, whenever we see something that looks like a callsign, we immediately go to QRZ.com to check it out.  And sure enough:

73, Pete, K2PS