I thought the whole point of mines was to use surprise.
COH has created the idea that mines are supposed to be ambush weapons, but that's not generally how they were used.
You can use all other mines as ambush weapons, but that's because they possess a characteristic that RL mines didn't:
They have magical IFF that won't go off if a friendly triggers them.
Usually you employ mines around your own positions and you want them marked for a number of reasons:
1) You are going to be spending more time in close proximity to that minefield than the enemy are and when you are doing a night movement you don't want to make a mistake and blunder into your own unmarked field. And even if you know where you laid the mines and are comfortable with that if you get relieved by another unit they need to know where you put all the mines or they will be really upset with you.
2) The purpose of a minefield is not to inflict casualties but to channel movement and delay an enemy advance. Vielsalm demonstrates this principle beautifully, because everybody knows that there is a huge minefield in the middle and this channels early combat to either side. Marked minefield works better at that than an unmarked one and is further why mines are overlooked by enemy weapons so they can't be cleared easily.
An obstacle not covered by fire is not an obstacle
3) You might want to lift your own mines and relay them somewhere else after the front moves so it helps to know where you put them and the pattern you laid the field out with.
Now, blah, blah, realism/historical accuracy etc, but if one considers how mines were used IRL and accepts that this game is at least inspired by reality then decisions and game play make sense.
If anything one could equally argue that all mines should be marked, except possibly those laid by Soviet Partisans