One of the stealth skills gives you a "radar" where you can sense, when you activate it, enemy troops. There you can see them, even when you don't have LOS, as triangles facing a direction and color. If red, they are on full alert, and if they know where you are they will come after you, yellow, they are aware something is wrong and are looking for something, cyan, all is fine and peaceful.
If you upgrade your skills enough to get a mastery in this field (not difficult at all), then it becomes from active to passive skill and is on, all the time. Which is what works as a warning system or whatever.
For me, this makes a lot of sense. You only get it when you deploy points on stealth, since then is the time when you most need it. If you go all guns blazing, you don't care THAT much. Still, as I said it is cheap, so you can invest in it if you require it even if you don't play as a stealth character.