The Mine Detector item can be found at any starting location. It is a misc item and can not be equipped. You can toggle the Mine Detector on and off in the Action Menu of FROST. You can set a hotkey for the action menu in the MCM menu of FROST.
The detector consumes a "Mine Detector Charge" each minute, and sixty charges are provided at each starting location.

The detector can detect all mines and certain traps (like the electrical arc), and makes a sound every few seconds while a mine is nearby. When the detector finds a mine for the first time, it will also play a special detection sound, and when you are too far away from a mine or trap, it will also play a sound.

The mine detector and its charges can be crafted in the Tools section of the Chemistry Workbench.
Mine Detector Charges are in the Ammo section of the Pip Boy because the PipBoy UI does not correctly display the weight of the Mine Detectors Charges if they are not an ammo type objects. This is a vanilla issue, and has nothing to do with FROST.
A mine detector is also present when you get to Far Harbor.


Known bugs/problems:

• Mines that disappear through the "deathloop protection" mine despawn feature of the game engine will still be detected by the mine detector, but can't be found. It is not possible to tell the scripts to detect if such a case is present or not.
• Mines that were placed by the player will also be detected, there is no way with Papyrus scripts to check if a mine is owned by the player or not.