The Ace Key is available in the "desperate times" scenario of Resident Evil Outbreak File #2.
On EASY, NORMAL and HARD, the key can be found in the Kennel on the third cell where the player can find Tony dead, after placing all five plates in the main hall and therefore starting the countdown for the zombie invasion.
On VERY HARD, however, the key is found on the other side of the Waiting Room where the player starts, Entrance Hall-relative, before the countdown starts. The player must push the crate from the 2F East Hall side of the barricade to obtain it on the Entrance Hall side.
The Ace key can open the safe on the shelf on the 2F East Hall side, in the Waiting Room. The content of the safe varies depending on the difficulty;
On EASY and NORMAL, the safe will contain a fully loaded MAGNUM HANDGUN and a Submachine Gun Magazine.
On HARD, the safe will contain just contain a fully loaded MAGNUM HANDGUN.
On VERY HARD, however, the safe will contain the Ruby Plate, which is required to solve the statue puzzle.