I got this warning once when trying to take of from a grassy park, no idea why and second time when trying to take off from a concrete deck with I assume lots of reinforced steel inside?? First time I just moved the craft about 5 m further and that was enough to satisfy the compass. Second time I had to go to a completely new spot away from that deck. The warning would not go away while the craft was on that deck..
Grassy parks can be misleading, as they may have ferrous water pipes, sprinklers, drainage pipes....just a foot under the pretty grass. I've launched from parking lots, with rebar, by flying drone from grassy/dirt areas where bushes are planted....often no ferrous stuff under the plantings.