See players through walls as red shadows?

In Call of Duty Advanced Warfare, sometimes I can see enemy players through walls as red shadows.

They come and go, sort of long blink type of "coming and going." The effect lasts for a while, I think it must be 10-15 seconds.

Obviously this is some kind of effect created by a player.

My question is what item/perk/whatever it is that creates this effect?

I thought it would be the Threat Grenade, but I have yet to figure out how that one works, I shoot it into the area, and it detonates but it doesn't seem to show me any threat indicators. Would it have to detonate and "infect" enemy players?

You are correct in thinking that this is because of the threat grenade.

This picture beautifully displays the effect of the threat grenade, temporarily displaying red over the players, which can be seen through walls. The enemy must first be hit with the threat grenade before you can see the effect.

Threat grenades are new to Call of Duty: Advanced Warfare and can be useful in revealing enemies through walls – painting them a vivid red on your screen so you can track their movements.

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