It really seems ridiculous. First you encounter the mine in whatever way. Second, you have to move the engineers close to the mine, because for some reason the mark mines command isn't available at a distance. Third, once they've reach the mine you can order them to mark it. Then for a large mine field, you have to repeat steps 2 and 3 for every individual mine!
The engineers dont have to be near a bocage to blast it. Its just a movement order. Why can't the mark mines command work the same way and order engineers to move through a field, locate mines, and mark them all in one order.