Wallhack.  https://youtu.be/NyzmKOia2iQ

00:15 on the minimap there are enemy showing up toward the entrance, NONE inside.  He runs up the stairs and instantly turns to the enemy inside. Turns and is ADSing through a box at the enemy.
01:55 aiming thru the wall into the cell.
02:10 again knows the guy is there.
02:20 knows he's there
02:25 follows the guy thru the wall continuing to shoot after he ducks in cell
05:00 knows the player is behind the pillar
05:15 sure, there are lasers to give away enemy existence, but he prefires aiming right at 'em.
05:29 tosses a nade at a player he has no reason to know exists, then shoots him
06:30 prefires at an unseen enemy
06:50 he pulls up short and turns to look through the wall!