What if the people attacking just HAPPEN to have their repair guns with them? Visible mines get disarmed.

If you put them over the door, on the inside, they always find their mark(you must pass through the door, after all), and cannot be disarmed before they blow up, as they are blocked from outside view(but not from kills)

So in other words, you end up with a mine that looks as if there's no mine there, and they just run right in, oblivious to the fate over their heads...and even if they did know, they'd have no choice, as they cannot be disarmed until you actually go inside-which makes it a moot point.