The inventory description reads as follows: "enc_weapons1_ammo_ammo-5.7x28-fmj"
It appears to be ammo, given that it has "30" on it.
This is an ammo stick for the FN P90 cut weapon and ought to contain 50 rounds. Similar calibre ammo may also show up in the Merc locker stash, but this has a different (erroneous) icon since it is ammo of a different grade. (No instances of the FN P90 show up on an unmodified gameboard)