Continuity mistake: Mr Freeze's ship just appears out of the clear blue rather than having been anchored all night when you can play "In from the Cold."
Batman: I am vengeance. I am the night. I am Batman.
More trivia for Batman: Arkham Knight
More questions & answers from Batman: Arkham Knight
Answer: In Batman: Arkham City, Mr. Freeze's gun was taken by the Penguin, which he used in an attempt to stop Batman. After beating the Penguin and the eventual closing of Arkham City, the gun was then sent to the GCPD evidence room.
But Mr Freeze got it back for his fight with Batman, remember?
Rob245