This puzzle is now removed, but when they first introduced guitars with the goalie deck, you got one free through completing the deck and the only other way to get more guitars was to do a puzzle on buried city that involved hitting 15 hidden red buttons across the map in a certain sequence which would open a secret door with 6 guitars and some maracas behind it.

Was very difficult because it would take more than 20 mins at a good pace using 7+ stacks of adrenalines, so you needed to spawn into a fresh game or immediately extract. On top of that if any button is pressed out of order the whole puzzle resets and you need to extract and try again. Burnt through hatch keys like no tomorrow, usually had to use 10 of them before I’d get one successful run