In Halo 4 matchmaking (War Games), I have occasionally seen an enemy on the minimap get a red radar-like blink, with an accompanying noise. What causes this blip?
This is someone with Promethean Vision spotting you using the ability.
This ability is one of the "Armor Abilities" that you can choose once you have Spartan Rank 1 and "buy" it with 2 Spartan Points. When activated, the ability changes your HUD and sends out an periodic pulse that highlights enemies, even if they are behind walls.
If you have been "pinged" with this ability, (and therefore you have been seen through a wall) you will be notified with an expanding circle on your radar where the enemy was when they saw you. This is your warning that an enemy with Promethean Vision has seen you.