Ah yes. Good point.
Likewise, of course, we'd know if the magnetic or geomagnetic pole had moved a large distance unexpectedly.
For one thing, I'd have walked over a cliff in the mist a couple of weeks ago, since my route-finding was based on the assumption that my compass was pointing in the usual direction. Even in these high-tech days, ships and planes still carry magnetic compasses, the behaviour of which is regularly checked. The magnetic north pole has drifted only about five degrees in the last fifty years; still enough to mean I've had to slightly adjust my compass over the decades, but nothing surprising or unusual.

Grant Hutchison