Walt's encounters was over IYK, which happens to have a radar right on the airport. I think this provides coverage up the Owens valley toward Bishop.

On Saturday, October 14, 2017 at 6:03:20 AM UTC-7, wrote:
After passing by I checked my PowerFlarm and found no transponder target.

For powerflarm to see a Mode C or Mode S transponders, the target needs to be interrogated by a ground station or another aircraft.
The powerflarm itself does not interrogate does it?