I just had a long, difficult scavenging mission completely ruined by what I call an invisible marksman. Triton Marksmen in mist that will not even reveal when a soldier is not only standing right next to them, but even moves a tile while right next to them. Utterly maddening and just f-ing BS.
throw a granade or two there. 
I was out. It was a 3 man + Scarab squad near the end of a hard scavenging mission midgame with B squad members. In other words, a challenge under normal circumstances.
I can handle those Tritons so long as they can be revealed by moving next to them. When it gets REALLY frustrating is when you can stand right next to them and they still don’t get revealed. I think it’s something to do with their exact stealth score? Don’t understand the stealth system well enough to tell.
Or, yeah, you can just blanket the area with grenades, which is probably the easier way to do it.