Most people are going to prep and stash in familiar territory,,,most people wont be caching in uncharted territory, so it isnt going to be an issue unless the person were to die, in which situation, some other family member should be in the know anyway.
I use GPS in my business, and I find it to be very accurate, I can drive a pin and then later return to within a couple feet of it, close enough to find it again with the metal detector, so, just common knowledge and as you say grid map, I could have gotten to within detection distance anyway. Ha. Then send the map home with your son or daughter for safe keeping, lest it fall into enemy hands..........ha. Some people just need a check up from the neck up.
Whittington Center Life
Times are tough, ammo is expensive, there will be no warning shots.