Or maybe it could be activated alwasy without the possibilty to switch it off. If you assign it to a useless key you never use (in my case "O"), press it once to activate the 3D map, you're done!
Same problem I and many others willkeep forgetting it.
But you only have to do it once. Like this:
1) Open FH2 game
2) Go to Options/Controls/Infantry
3) Change 3D key from Alt to "InsertUselessKeyHere"
4) Join server (empty one preferably)
5) Take scoutkit
6) Put S-mine
7) See if you can spot the red skull icons when you are close to your own S-mines
a) If yes: profit! Done, don't change anything anymore
b) If no: press the key you chose at 2) and try again. You should end up at 8a) now.
The game will rememeber if you had it off or on when you leave the server, even if you join a different server. So you don't have to remember yourself! The game does it for you!
And if you can't remember to do this once, then you should go easy on the pot, dude ;)