"Later, soon after take off, PowerFLARM indicated a target 200ft below me and quite close"
That is exactly what my test unit did when it detected my transponder!
Many pilots do not switch transponders on, including instructors. It is quite interesting to land after an aircraft which was not showing on my PCAS and ask them on the ground. Most come up with one of 2 reasons;
1 I like to keep a low profile - I don't want any trouble if I make a small mistake.
2 The only guy to get prosecuted for the Red Arrows bust was the only guy who was squawking, the others were never traced. (which is true).
I have pushed many times for a sensible harts and minds campaign to get people to turn units on, but such an approach has to take into account human nature not just shout idiotic fool.
Rod1