Seems like a weird objective...
Having it show you hidden mines through walls kind of defeats the purpose, but having it show you only ones you can seems redundant. Although, the latter could be useful if you already know where the other objectives are.
Mines probably won't be marked as objectives for you until you've seen them.
So you get line of sight on the mine, then get chased off before you can mark it. The mine becomes a potential objective, though, so you can use the objective wheel to get your bearings, get back to where you saw it, and use ironsights to mark it for the team.