This is available in Stage 9: "Slaughterhouse".
There are three instances where you will need to dodge cameras attached to the ceiling, thereby avoiding being detected by Leon and the screens getting turned on.
The cameras do not emit any kind of light or pattern, so you might not notice them and think they're background dressing - but they're not. They only see in a direct (invisible) line beneath them, so the detection range is fairly small.
- The first one is right after the first screen. There will be an enemy standing in a large room, with a door between you two. Lure the enemy into the hallway and kill him, then roll underneath it. If the large room's barrier doesn't go down and the screen doesn't turn on, you were successful.
- The second one comes much later in the stage. It's a huge, dark room with conveyor belts. Normally there would be a big boss-like fight in here, but you can bypass it entirely by rolling underneath the two cameras. Again, if the barrier doesn't go down and Leon ignores you, you did this right.
- The third one is right after the above, normally Leon will detect you and blow up the lab equipment at the bottom of the room. Roll underneath the cameras to stop this from happening. You'll know if you did this right because the lab will be intact and you can search it for the key.
You can always open the menu and pick 'Restart' if you messed up and need to retry a room.