Intravenous Stealth until it breaks

Visibility depends on light, and every step makes noise, so the shadows are a resource you spend. You shoot out lamps, flip circuit breakers, crawl through vent shafts and lockpick doors to find another way in. A bottle thrown down a corridor buys you a crossing. When a light you broke starts flickering, it still gives you away, so darkness is a tool with an edge on it.

The guards here do not walk a loop and forget you. Find a body and they alert their friends and start searching. Hear a footstep and they come to investigate. See you and they open fire, take cover, sweep the dark with flashlights and radio in what they found. Unarmed ones run off to warn the others. Once suspicion sets in, it does not fade back to "must have been the wind": the hunt stays on, and your careful plan becomes a live problem.

Stealth that fails into nothing would be a design flaw. Here, getting spotted flips the game into tactical gunplay: enemies with precise aim, cover discipline and numbers on their side. Camping a corner gets you killed. What works is shooting, moving, flanking, looping around through a window, turning the lights off behind you and hitting them from a direction they forgot about.