I'll answer this question. I have a ton of time flying gliders where this happened. The glider pilot was flying in a normal area for glider ops. There is a ton of glider flying in that area of the carrson valley. It is the soaring capital of the world. That is where the glider world altitude record was set. Most times of the year there is standing box that allows us to go up to FL600. I've been up to FL380 in a glider there. The glider I fly has a transponder and a radio. I talk to ATC, but as long as they stay in the box the are not required to talk to anyone. 16000ft in that area is low for a glider. Most pilots can get up there on a day with weak thermals.