Per the [Star Wars: Helmet Collection #2][1], the "flag" houses a holographic scanner and a range-finding sensor.

[![enter image description here][2]][2]

> **Targeting rangefinder (retracted)**
The helmet tracking sensors sent information directly to the helmet
displays. It gave Fett precise readings on the speed of, and distance to
any given target.

and

> Holographic targeting display When flipped down, the targeting display interfaces
with the visor to give tracking information on up to 30 targets at a time.



  [1]: https://starwars.fandom.com/wiki/Star_Wars_Helmet_Collection_2
  [2]: https://i.sstatic.net/nTnWd.jpg