Originally Posted by
Maoraigh1
My Mode S was being interrogated, at least some of the time
I'm sure someone will correct me, but being interrogated by a TCAS unit could cause this, and if the signal is weak (or trying to go through a mountain), it doesn't mean that your interrogation response was being received. I'm sure there are also some non-interlinked SSR stations around also (military?) which could interrogate your transponder.
I haven't visited fisbangwollop and his colleagues yet (despite flying from Prestwick) but I understand that although they do have visibility of a radar screen, it is as an advisory only - they normally use a map, some pens and a ruler.