Where are these traps? Here is what I'm thinking: I'm sure the signs of a good booby trap are very subtle and almost impossible to spot if you are just walking around, not expecting anything. It would depend on the trap, of course, but things like broken twigs, disturbed dirt, etc might be examples. These things can't really be done in ofp, since the closest you can do is to place a small object on the ground.
Maybe the best way to indicate a trap is to put them where things just aren't right. I guess that is how you would notice a trap anyway, but in ofp terms, it could be something like an unguarded breach in the enemy defenses, or an obvious doorway that isn't used, etc. You would need to warn the player to be on guard for boobytraps, of course, since this kind of thing isn't standard to ofp missions, but I think it could work very well if done right.
Let's say you are supposed to rescue some captives from some terrorists. They've holed up in some abandoned buildings for the night. Out front, there is a fire and a sentry posted. However, there is a door around the back, by the bushes, that looks unguarded. So the careless player sneaks in through that door, thinking he has got the best of the terrorists... he walks throught the door and BANG! He trips a wire, and a grenade blows up next to him. The terrorists are now alerted, and know exactly where he is, so if he is still alive, he likely won't be for much longer...