You can find this Safe when you go up the tower in the main mission. It's located near the Inhibitor so it would be very hard to miss. The hint "You're my 555 and I'm...." is completed by "666" - which is the code to the safe.

Make sure you don't turn on the tower before you get this stuff though. Otherwise, you'd have to go back up the tower (won't take long with a paraglider - you'll see if you don't follow these instructions :D )

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