From The Long Guide by Frankelstner - detailing stealth mechanics analyzed from the extracted game files:
"Cool" refers to the state of person or camera during stealth when he/it has no exclamation mark.
Once the person or camera has an exclamation mark or alarm goes off, there is no going back: The subject can never become cool again. I refer to this state as "uncool". Dead or broken subjects are not included.
Stealth - Alerts
Alerts are noise and instantly turn people in hearing range uncool. In the context of this game, alerts mean that neither the orientation of the head nor obstacles make any difference and that the person becomes uncool instantly without becoming suspicious first.
Human observers become alerted by the following:
- Glass breaking (range: 12 m)
- ECM feedback (range: 100 m)
- Gunshots (range: 25 m when enemies shoot, usually 45 m with player weapons)
- Gunshot impacts (range: 1/4 of the gunshot noise)
- Grenade/Flashbang explosions (range: 100 m)
- Trip mine explosions (range: 50 m)
- Taking fall damage (range: 2.5 m)
- Civilian screams upon becoming uncool and at various other times (range: 2 m)
- Civilian or enemy screams when experiencing moderate or heavy hurt or death (range: 6 m without Shinobi or Hidden Blade aced, else 0.25 m). Even handcuffed guards scream. Non-lethal melee attacks against civilians cause no scream.
Unless you shoot in the sky, every gunshot ends with a gunshot impact. The noise origin is the impact location. Technically, gunshot impact alerts are of the same type as gunshot alerts. As a result, gunshot impacts may cause alarm on day 1 of Big Oil, but they can also be used together with Control Freak to intimidate civilians.
When your gun has issued a gunshot alert (i.e. you have fired your gun), subsequent gunshot alerts by the gun are blocked for 1.5 seconds, unless the origin of the alert differs by more than 5 meters. Whether you fire your CAR-4 once or 15 times within 1.5 seconds to control civilians with Control Freak, they both issue just a single gunshot alert. There is a separate but pretty much identical block for gunshot impact alerts, where the 5 meters rule has a greater effect.
All player weapons have an alert range of 45 meters with the following exceptions: Brenner, KSP. MG42, R93, Thompson, Queen's Wrath, Lion's Roar, minigun, Lebensauger have an alert range of 40 meters. The Thanatos has a range of 35 meters. The saw also causes gunshot and gunshot impact alerts and has an alert range of 35 meters when not using up the blade (2 m if silenced), else 75 meters (8.5 m if silenced). The flamethrower has an alert range of 300 m. The grenade launchers and RPG do not cause gunshot alerts.
All weapons with an alert range of 45 meters have their alert range reduced to 1 m when silenced (thus their gunshot impacts have a mere 0.25 m range). Weapons with an alert range of 40 meters or less (except saw) have their alert range reduced to 0 when silenced.
Uncool civilians cannot hear anything but gunshots (and gunshot impacts) caused by the players. Control Freak basic fully intimidates civilians when hearing a single gunshot alert. When hearing too many gunshots of players without Control Freak basic in quick succession, untied civilians will stand up no matter their intimidation level (only Dominator aced blocks this). This being said, due to the 1.5 seconds cooldown it is virtually impossible for a single player to make a civilian freak out and flee.
Uncool enemies cannot hear anything but gunshots (and gunshot impacts) caused by the players. Technically, they were supposed to listen to explosions and glass breaking caused by the players as well. However, the function used by HE rounds, hand grenades and the grenade launchers explicitly claim that the cause of the noise is the group of civilians and enemies, but not the players. As a result, any alerts caused by that function are ignored by uncool enemies. Breaking glass also does not work correctly and is ignored by enemies. Thus enemies behave like civilians in that regard and will react to gunshots only (and maybe trip mines). Enemies prefer to target players whose alerts have been heard and may also decide to shoot due to alerts.
Stealth - Alarm
There are 4 different ways alarm can be raised:
- An uncool person (guard or civilian or gangster) calls the police.
- A camera is uncool.
- A pager is left unanswered or answered unsuccessfully.
- Map-specific triggers. Usually motion sensors (lasers) or alarm buttons.
Basic ECMs disable the first two triggers as long as they are active. With the ECM pager skill, alarm is raised only by map-specific triggers.
There are three ways to figure out if alarm has been raised:
- All exclamation marks suddenly disappear simultaneously despite the enemies still being alive.
- A text message appears on the screen. Some map-specific alarm triggers do not show any message at all (Rats day 2, Big Oil day 1, Firestarter day 1, Framing Frame day 3), or are otherwise incorrect (alarm buttons on Trustee bank).
- The music changes.
On Big Oil especially, if you are not the person stealthing it, the only reliable method of figuring out if there has been alarm is paying attention to the music, as there is no alarm message and the disappearing exclamation marks might be due deaths. Conversely, as long as you can see a single exclamation mark, you are still in stealth as no alarm has been raised yet.
Excerpts from 'Dispelling Common Misconceptions' Reddit /r/paydaytheheist post by KarateF22:
- Civilians are alerted by bags: They only care about body bags and only at close range. Guards, gangsters, and cameras do care about all bags, however.
- You can stop a camera alarm after detection with ECM or by killing the operator: False. Operators aren't even alerted when the camera is; once a camera fully alerts stealth is guaranteed to fail in 7 seconds.
- Grenades sound the alarm on Big Oil Day 1: Hilariously they do not. Gunshots and the saw still will however, so be wary of that.
- Shinobi Ace is useful: Kinda. It works as claimed for clients in a game. However, there is currently a bug where hosts always silently kill targets, regardless of having the skill or not, which makes this much less useful if you host frequently.