1. Play either the Scuba Showcase or COOP 7 Headhunters.
2. Try to find an enemy far away.
3. Either wait a bit, or make a sound either by a gunshot or something else. (Mortar fire in the Scuba mission tends to get you spotted afterwards quickly, due to the alert.)
4. Watch as enemy AI spots you quickly and shoots at you, even if you're in a stealthy position/spot.