From Halopedia, the Halo wiki
The OPTREX module is a modular helmet attachment employed by the UNSC. The module can be employed on the Mariner helmet, and contains an enhanced optical suite for detecting polyband thermal and chemical signatures.
The OPTREX module is available as a helmet attachment for the Mariner helmet. It is one of a number of pieces of gear cut from the release of 2011's Anniversary Map Pack and brought into the Reach component of Halo: The Master Chief Collection in Season 5: Anvil.
List of appearances