From Halopedia, the Halo wiki
A Threat Marker is a field marker attached to a firearm, to help track targeted enemy personnel for around 4-5 seconds it can even target invisible enemies (requires some visibility to lock-on) and it can target up to 3 enemies at a time as well as targeting all targets on board of a vehicle and show the distance between them all. It generally takes the form of a small circular device with a beam of light coming out of it, placed on the side of the weapon.
Known Applicable weapons
The following weapons have been known to utilize this attachment:
List of appearances