Sniper Position

The Sniper Position was an event that occurred during the Christmas Eve incident, which led to the murder of a SWAT sniper aiming for Batman, and the shooting down of the SWAT chopper Air One by Deadshot, one of the eight assassins after Batman's head.

Before Arkham Origins Incident
Sometime after the Joker took over Black Mask's operations, Joker proceeded to hire eight assassins (Deathstroke, Copperhead, Firefly, Bane, Electrocutioner, Lady Shiva, Killer Croc, and Deadshot) with a bounty of $50 million on the vigilante Batman's head.

Arkham Origins Incident
After Batman deduced Deadshot's likely involvement in causing an earlier helicopter crash, he went over to the sniping position he found from the bullet trajectory. After scanning the discarded round, he found that it matches the rounds used by Deadshot by comparing it to the National Criminal Database, although the trail ended there. Batman then discovered a set of numbers on this casing for a frequency, with Deadshot revealing that he had anticipated that Batman would find the round and demanded he come to Gotham Merchant's Bank, also threatening that he'll kill any hostages he finds to ensure Batman came over.

Casefile details
Batman: The trajectory analysis will lead me to Deadshot's firing position.

Batman: This must be where the shot originated. I should scan the area.

Scan of bullet casing

Batman: Just as I suspected. The shell casing matches others in the criminal database. And they're all linked to Deadshot. But his trail ends here. Unless... There are numbers on the bullet casing: 451,326. It looks like a radio frequency.

Deadshot: Hello Batman. I see you decoded my message. Well done. But I'm afraid playtime's over. Come meet me at the bank so I can put a bullet in you and collect my prize. Oh - and in case you're thinking of backing out - I've got a hostage and a very itchy trigger finger.

Trivia

 * Although this casefile is technically part of the Helicopter Crash casefile, the game treats it as a different casefile due to Helicopter Crash being considered a completed crime scene after scanning the downed SWAT officer. As such, this is the shortest case file in the game, only needing one scan to complete the casefile.