Mark

Marking is a negative status effect that allows the marked player to be seen by their opponents through walls.

Appearance
Marked users are marked with a dark translucent squid shape that wraps around an opponent, with a line leading to each member of the team that marked them, though each member only sees the line that leads to themselves.

Causes

 * (Splatoon 2 onwards)
 * (in Splatoon)
 * (in Splatoon)
 * (in Splatoon)
 * (in Splatoon)
 * (in Splatoon)

Similar abilities
There is a similar effect caused by the Haunt (Splatoon 2 onwards) and Thermal Ink main abilities and Sting Ray, where the bearer can see an enemy's silhouette through walls, after the enemy splatted them (for Haunt) or was hit by ink from their main weapon (for Thermal Ink).

Also, Flooders and Grillers locate the player with a red laser that locks on to them. Grillers can track the player anywhere, but Flooders require the player to be within their line of sight.