Good idea about the compass. Personally I cannot think of anything underground that could do it. All the local power lines are above ground. Its all protected land.
But its funny though as the same thing happened to my Heli when I flew it quite a way off where I was today I am seriously wondering about the Radar Place down the road. You never know what they get up to in there.
If you read this you will see what I am on about RSRE, DRA, DERA and RADAR Especially if you read the second from last paragraph.

If you read this you will see what I am on about RSRE, DRA, DERA and RADAR Especially if you read the second from last paragraph.