I'm starting to suspect the the type of ground you put the mines on also plays a part in how "easy" it is for the enemies to spot them. When I put down mines in the random encounter maps thay usually goes off without the enemies seeing them, while when I put it on roads, in cities and similar flat ground they spot them right away.
Since the maps have a camo "bonus" feature on some surfaces for the characters, I think the same goes for the mines. But this should be tested more to see if my suspicion is warranted
EDIT:
It would also explain why the Hidden value in the mineinfo file make them work better when set to 0, I think that disables the environment check. They should've added a camo entry in that ini file instead of using a global value for all the mines, that would make fixing it a bit easier.