I'm also unconvinced without even looking at the data. It's been a while since an incident was attributed to a flawed compass calibration. Likewise a flawed calibration that was caused by a particular location. These things just never happen.
I'm guessing it was a rivet, zipper, buckle, etc. Doesn't have to be large since the P4 compass is in the leg.
This. The obvious explanation is that there was no improper compass calibration.
This is to check that the Yaw value is correct. Has nothing to do with checking the compass calibration.
So if the radar pointer is off how do I correct the yaw value? I did get a compass warning before take off on the beach, so I moved the backpack and AC 10' away and the compass warning cleared.