In Call of Duty Black Ops 6, you will come across a black light (on a table) that will reveal hidden messages in the environment. It can first be used to check for footprints next to a piano on the first floor of the safehouse. When you explore the area with said black light, you will notice some markings (Cyrillic letters) behind the painting, on an adjacent wall, next to windows, etc., all around the room. They are all connected to the piano and if you are not sure what they mean, here is the solution to this Black Ops 6 puzzle.

You need to solve the piano puzzle in Black Ops 6 to open the basement door. To do this, look at the symbols on the walls, as they should be numbered in this order:

Now go to the piano and press the keys to see the symbols above it light up as well. Your goal in solving the BO6 Piano Puzzle is to play the notes in the same order as the wall markings. Note that there is a chance that the keys for players will be different, but the solution format remains the same.

Doing this will open a door that will lead you down to another door, which will lead you to more puzzles in BO6. Solving the piano puzzle and lighting the boiler will give you access to a basement, where you will need to unlock a door, hack the computer, open a door, and pick some locks.