Fiber-optic probe
From Halopedia, the Halo wiki
The Fiber Optic Probe is a stealth tool used by UNSC military personnel who are wearing some form of armor. Fiber Optic Probes are small wire-shaped devices that are plugged into an outlet on the helmet of the armor on one end, so that the wearer can see through the other end via their Heads-Up Display.[1]
Fiber Optic Probes allow the wearer to see around corners or through small spaces without having to reveal their positions by looking up and exposing themselves to enemy fire. Some of these probes measure at least 3 meters long [2]
Apparently, these are used by both ODSTs and Spartans through their Body Suits and MJOLNIR armor.
