Some enemies or even non-hostiles affiliated with the hostiles can call for reinforcements. You can identify the NPCs capable of calling for reinforcements by Profiling them. If a call was successfully made, their allies will arrive shortly and will begin to sweep the area to find you. You can stop an enemy or NPC from making the call by attacking them, using melee attacks, or using the Distract Hack to interrupt the call. (You can do the same thing against witnesses) Furthermore, there are robots and security systems that can automatically call reinforcements so don’t get caught unprepared. The same thing applies to stopping witnesses trying to call the police.
There are several factions in the game. While most are neutral toward one another, some of them are enemies. It’s not surprising to witness some gang members attacking their rival gang’s territories or them engaging the police. You can also force a conflict after getting the Gang Attack and APB hacks to call a random gang hit squad or police force on the target. Take note that when there’s a firefight between the police and the gang members, engaging the gang members in order to “help” the police will just make them hostile towards you. If you want to lend a hand to the police, you can do so indirectly by using the distraction hack or remote grenade detonation to gang members.
Here are the different criminal factions in the city:
Law Enforcement factions are friendly towards one another and will help each other in case they’re engaged at hostile factions or against you.
Heat increases as you commit crimes or engage other factions in combat. Notice the bar under your minimap. The markers there represent your heat level and the higher you get, the deadlier the response deployed to you. Furthermore, other players may join the hunt alongside law enforcement units to take you down. The color of the bar under the minimap will also change accordingly.
These are red-colored areas in the map. They are guarded by armed members of the faction and/or by dogs. Entering one will automatically put Marcus in stealth mode, crouching and wearing his face mask on. Faction members will be aggressive toward anyone who enters the area, including innocent passersby. Affiliated civilians inside the area will also call for help if they spot you. If the enemies spot your drones inside the restricted area, they’ll attack them.
Restricted areas usually contain money bags, collectibles, research points, and key data so it’s worth checking some out. With the proper upgrades, it is possible to clear restricted areas and collect whatever collectible is in there using your drones alone, as long as you right outside the edge of the restricted areas.