Enemies are sneaky. Turning on your Thermal vision makes it easier to spot enemies by exposing their heat signatures. The Thermal button appears right next to the reload button whenever you have a Sniper Rifle equipped. The Thermal Vision will run out after a few seconds but it can be upgraded in the Weapon Upgrades Shop to last longer.
You can also use an Auto-Spotter (search the FAQ for Power-Ups to learn more) to highlight both enemies and friendlies in the area. Auto-Spotters do not have a duration timer within a Mission.