Mechanoid turrets seem to be able to sometimes lock on to pawns that are out of sight. It's never extreme, like being able to see into a building, but if a pawn's a couple tiles away from the edge of a wall (not at the edge, I know anyone can be locked on to if they're in cover at the edge) the turrets lock onto them. They never shoot, but it makes shoot-and-run tactics impossible because the instant they pop their head out of cover the pawn's hit by a barrage since the turrets have already done the shot cooldown. Combined with the clusters high-power and insane accuracy (Which is an issue of itself, it makes cluster assaults almost hilariously dangerous in my games), as well as mortars being horrifically inaccurate even with a spotter, it means that there's only one tactic for them and that's hitting all of it simultaneously with a load of explosives which isn't cheap and is, frankly, pretty boring.