Signal tracking device
From Halopedia, the Halo wiki
Scanner
A scanner is a kind of UNSC detection equipment, seen being used in part 2 of the Arms Race video trilogy. The scanner appears to be used for displaying the location of a tracking signal. It is possible that it is used for other purposes, although none are shown.
The scanner appears to have a flat panel display, and emits a beeping that gets faster the closer it is to the target.
Trivia
S. Hartley of Delta Team was seen using a Scanner in Part 2 of the Arms Race Video
References
Halo Arms Race Part 2