Requires DAHM/DorHud: https://steamcommunity.com/groups/dahm4pd/discussions/3/490123197956419203/

This mod adds ! waypoints above civilian's heads who are at risk of running for the alarm buttons (so, civs who are not surrendering, and civs with less than 95% submission). It works perfectly as host, and simulates the submission checks as client. However, it can only simulate your own shouts, so there will be a lot of false exclamation points as client, but it might semi-accurately show your own little area at least.

It is perhaps a bit cheaty but the information is not much different than Payday 2's magic question marks and cellphone icons. If you want to improve your abilities at No Mercy Stealth without using the mod, I'd still suggest using it in practice to learn more about the mechanics.

Details about how keeping civs down works:

The main mechanic is a "submission meter" that civs have when they're in the "surrender" logic state. This state is entered any time a civ is intimidated by your shout and falls to the ground. When they enter this state, they're randomly given a submission max between 60 and 120. Shouting adds up to 60 submission to their meter, so you'll almost always need two direct shouts for them to have full submission. At 0 submission they'll stand back up and enter the "flee" state.

Shouting works on any civ in the area in your field of view, and a line of sight check is done as well (so, civs touching your back will not surrender when you shout, but civs on the very right edge of your screen might, unless a wall is in the way). Because of the line of sight check, this can cause issues with civs whose head are clipping into objects, and it also means that jumping can help reach civs behind the desk.

The delay between your shout and when they go down can tell you how effective your shout was; the shout will always have a primary target who goes down right away with 60 submission added. If they go down after a 1-second delay, they likely have very low submission (10 or less).

Submission will only decrease, at a rate of 1 per second, if all heisters are far away from the civ (>700 units) and the civ isn't in their field of view. This check does NOT do a line of sight check; if you are halfway across the map and a wallhacked civ would be in your field of view, their submission will not go down. This means you can help your teammates out greatly during No Mercy by looking in the direction of their civs while staying close to yours. In addition, if they're right near the center of your view (focus > 0.8), their submission will increase by 1 per second. This check seems to occur per heister, so a team could increase it by up to 4 per second just by aiming at a civ.

Civilians with a submission meter less than 95% full (or not surrendering) can be selected to instantly get up and run for an alarm button. This starts a few seconds after the 7-second camera countdown finishes and the civ does not even have to be alerted. The closest available civ will be chosen, although this seems to be "closest to an invisible map element" or something, because for instance, on Normal with no civs alerted, it will always be the same doctor in the room behind the desk that will be chosen to push the button. And civs also seem to have specific buttons they're allowed to push, for instance, the white coat woman that can spawn in the elevator room will push the elevator room button as soon as possible, yet men there don't.

That 95% requirement means that just a few seconds with no one near/looking at a civ can allow them to get back up and run for a button; try to keep large groups of civs in your field of view during interactions, especially those right at a button. Also, surrendered civilians do their submission update function extremely infrequently (only one civilian does their update per second, so it gets worse the more civs there are) so it only takes one missed update for them to become a potential alarm hitter.