I don't know, but most likely something was having an effect on the compass while on the ground, showing normal compass readings. Then 100ft in the air, that effect no longer had an impact on the compass. I had a fly away with the Inspire1 where after 5 mins of perfect flying and more than 250m from take-off spot it went nuts, flying at full speed away from the our pilot until he switched to atti.Had flown it 4 other times that day in same area and I always check the MOD value. All were OK. Interestingly, it got the Compass warning while it was 100ft high in the air. Why would it go wrong then?
Did you see any "toilet bowl effect", where it drifts in circles that get bigger and bigger?