In Dying Light 2 Stay Human, there is a safe inside the prefab military building on the dock south of St. Paul’s Cathedral. The safe code for this safe can also be found inside the same building, specifically on a shelf above the desks to the left of the door. Select the security code in the Memorabilia section of the Collectibles tab and you will see that it says “End of WWII [EU]”. Like the Poland Baptism security code, this security code requires you to know (or Google) a piece of European history. The code is the date World War II ended in Europe, otherwise known as VE Day or Victory Day.
The end of WWII in Europe came on May 8, 1945, but since Techland is a Polish developer, you have to format the date in the European way. So the day comes first, then the month, then the year, i.e. 05/08/1945. Saint Paul Island (End of World War II [EU]) the security code in Dying Light 2 is therefore 85-19-45.
Related: What is the security code for St. Thomas Church in Dying Light 2 Stay Human?
Use the code on the safe to open it and you’ll find a red duck inside. But you might find that you can’t pick up the red duck. This appears to be because red ducks are (or at least were) used to unlock the Bicycle Parkour Challenge easter egg, which was removed from the game in the 1.2 update. It’s currently unknown if Techland intends to release this Easter egg for a later date.