I had two cases this weekend where I found the spot and the map was vibrating with no chest in sight.
One was in the wastelands about two o’clock on the round lake of pink liquid. It seems to be on a ledge that is not accessible. I got to every location near both on the same level and below. The map was going nuts but no chest.
Second was in Highstep in the location of the new farmhouse. The map goes nuts when you are near one of the houses. I think it is inside.