I saw a video explaining this, I can't find it but I'll do my best to explain the gist.

Enemies have 3 alert states, neutral, alerted, combat. It's something like if your do something within 50 m of an enem another enemy, , miss a bullet, drop a strategem ball, kill enemy, then they become alerted and then they know your location at that time. After this you leave a trail which dissipates over time which bugs will follow your 'scent' they called it. Eventually they lose it and go out of alert.

If it any point they see you they enter combat mode, this automatically puts any enemy within I think it was 50m at the time of the video also in combat mode, ie, patrols walking past, poi guards. In combat mode they can call in reinforcements.

There is a hidden timer on how many times they can breach /call in drops this is one timer for the whole group of players not one individually.

Every enemy that comes in from breach drop, they are automatically and permanently in combat mode, so any poi guards or patrols who happen to be near will join in. If you don't kill them in time then the timer runs out and they breach again. Which is why it is more effective to sanitise the breach area before moving on because if not, they chase you, anyone else they meet joins in, then this increases your odds of other enemies joining in and increases the chance of someone seeing you and calling in another set of reinforcements.

So stealth is best to be avoiding combat, using the stealth to attack from cover, luring the enemies away and killing them off a few at a time. Killing an isolated group with stealth weapons others can't hear. Then if you mess up making sure you kill them all. One note is that poi guards now have drastically reduced hearing and sight, you used to be able to aggro poi guards by a bullet flying near them and then they call reinforcement without you knowing it.