I'm not sure if this is a mod or a built in feature, but how do I tag enemies like this this image:
And here is another image without being zoomed in on the scope:
Best Answer
This is the "track targets" effect that is described on the Recon Scope weapon mods, under the "Sights" modifications of weapons. By targeting the enemy through the recon scope, the red diamond will appear above them.
As pointed out by Yorik, the Recon Sensor modification for power armor helmets will have the same effect and mark enemies when aiming down the sights of any gun. More information, including materials needed for this mod, can be seen here.
The diamond seems to persist above the enemy until it is killed, meaning you can track the enemies with the recon scope gun and switch to another.
The highlighting is part of the Scrapper perk, so there is no way to get rid of it once you have the perk. All it does is highlight items so you aren't losing out on anything.
From your comment it seems you are confused about it's functionality. Highlighting a certain resource like leather or copper won't stop you from getting other components when breaking down an item. Say you highlighted leather, a teddy bear breaks down into leather and 3 cloth everytime you destroy one. Highlighting one component doesn't stop you from collecting the other ones.
Your ability to sneak is affected by 8 different factors:
Equipment weight: Although you said there was no difference, there is a difference if you walk. The heavier the armor or weapon, the further you'll be heard if you move. However, if you stand still, your armor weight won't affect you at all.
Light sources: Light sources also influence your ability to sneak heavily. However, this only affects you if an enemy has a clear LoS on you. If you are in a lit room with no enemy being able to see inside, the light does not affect you at all.
Your agility skill: The more agility you have, the more difficult you become to spot when sneaking. This affects you in both moving and not moving.
Perception of the enemy: This is also a huge factor which both affects you moving or not moving.
Distance: Obviously, the further the enemy is away, the hard it will be for him to spot you.
Any form of contact: If you touch an enemy, he will immediately spot you.
Loud Weapons: Any form of attacking an enemy will make them spot you. However, silenced pistols, melee weapons and grenades/mines will not break your stealth, although it will cause the caution alert if you do not kill them in one hit. Grenades always cause caution alert if there are any alive enemies in the area.
Special gear: There are several modifications like shadowed which can improve your ability to sneak. Also, gear obtained from legendary enemies can have a stealth-improving effect on you.
Your pip-boy (includes the torchlight) do not affect stealth at all as you already found out. You can also listen to radio while sneaking and nobody will notice.
Best Answer
This is the "track targets" effect that is described on the Recon Scope weapon mods, under the "Sights" modifications of weapons. By targeting the enemy through the recon scope, the red diamond will appear above them.
As pointed out by Yorik, the Recon Sensor modification for power armor helmets will have the same effect and mark enemies when aiming down the sights of any gun. More information, including materials needed for this mod, can be seen here.
The diamond seems to persist above the enemy until it is killed, meaning you can track the enemies with the recon scope gun and switch to another.