Sometimes during combat in the dark zone an npc will get a vertical red bar around them (similar to loot but red)... What does it signify?
The vertical red stripe is the first phase of a successful pulse. After the vertical red line appears a red outline will then appear around the character. You will notice that if you lose line-of-sight on the character the character model will be replaced with red blocks that roughly represent the character's location.
This affect can come about as a result of using the "pulse" skill or by successfully hitting an enemy with a head-shot with the "precision" talent enabled for your character. When an affect is applied to a target after a successful attack this is usually referred to as a "proc."
Also, I am not sure if it is by design or a glitch, but I have noticed that this "headshot pulse" or "pulse proc" can sometimes be activated even if the player does not have the appropriate talent enabled.