How the F do you stealth?

I was wearing the light scout armor, was down a hill, and even then had multiple layers of cover between me and a bot on a machine gun turret. I was planning my attack approach and wasn’t moving, nor had I shot or thrown a stratagem. Out of nowhere, he hops off the turret and pops a flair and now I have two spaa tanks and fucking factory strider up my ass, along with 8 berserkers that dropped directly on top of me. I’d understand if I walked past them, or was running nearby, or did something hostile, but I didn’t. I was just trying to clear the fortified extraction.