Even with PowerFLARM on "stealth" pilots may soon have another way to find the next thermal. The DARPA "One Shot" initiative will develop a rifle scope which uses LIDAR to detect and correct for crosswind "windage". The Israeli Soreq Nuclear Research Center has already been granted a patent on the LIDAR technology.

http://www.accurateshooter.com/optic...ar-laserscope/
https://www.fbo.gov/utils/view?id=1c...0d40d64c7cc251

Rotate the 'scope 90 degrees and you have a thermal detector which directly reads vertical velocity. Presumably, if the system can be miniaturized to fit in a rifle scope and be powered by man-packable batteries, fitting it into a sailplane shouldn't be that difficult - costs aside.