So we just tested this in custom games - they're very rare (between 10 of
us we only found 3 in a game with spawn rate at 3x). It seems like you have to
fire them into the air - the first two screenshots are when we fired it at
another player (it did no damage), and no crate arrived.
The flaregun goes into your pistol slot (though it's controlled by its own
slider in custom games) and contains 1 ammo.
The crate contained two weapons and a suppressor (which was already looted
when I took the screenshot). Since we only tested 1 crate it's not clear if the
AR+Sniper combo is deliberate or just lucky.