Hidden MineField

By dugger, in Tide of Iron

A friend and i just finished playing a scenario that the germans recieved a card called "Hidden MInefield". My question is how is it hidden if you place the minefield token on the board. I was considering making a house rule that during the opponents turn, you get to pay the command and place the minefield as a interupt. What do you think about this and if it's no good do you have any good suggestions.

Thnx.