One of those secrets is the Epsilon Outfit which you can unlock in GTA Online. Not a lot of players are aware of this outfit though, since its release with the Arena War update.
In order to unlock the secret Epsilon Outfit you simply need to tip the Nightclub Attendant in the restroom 575 times ($1 per tip) for a total of $575. The process is quite lengthy but worth the effort!
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Fun fact: sometimes when you pass by the Nightclub Attendant, he says "Kifflom" which is a word used by the Epsilon members to greet each other.
The only general requirement in order to be able to unlock the Epsilon Outfit is a Nightlcub. You either need to own one or head over to your friends' Nightclub. After you have unlocked this Outfit, it will be accessible through any one of your wardrobes.
Unlocking The Outfit
There are three main milestones you need to reach while tipping in order to unlock this outfit. Upon reaching each milestone, you will get a message pop up in the bottom left corner of your screen, just above the mini-map. The milestones with their respective messages are:
Accessing The Outfit
Once you have tipped $575 to the Attendant, you can go to your wardrobe and access this Outfit under Outfits: Special. it will be called Epsilon Robes.
And that's how you unlock this special secret Epsilon Outfit. Kifflom!