Apparently it's not connected to any of questlines at all. I managed to get there once, but sadly i can't do that again since i'm not sure how it works.

What i did...
1. Completed ALL questlines
2. Have got all bonuses
3. Killed a lot of people (probably all of them)

I tried to that again, and it's not working. I wish to know if there is anyone else who managed to activate it.

And yes, just a note: There is a guy in one of the buildings, which says "I can't let you in there". The door behind him is the entrance to that area.

I'll try to play around and attempt to find a way anyway

... alright, i found it accidentially. It was stupidly easy to do.
So, in order to unlock this secret, you need to wear sunglasses. Once you start wear them, police occupies that building with the guy i mentioned. You'll need to kill cops and enter this door.

Sunglasses? How to get those?

You know where is beach placed, right? So basically there is backyard (of "XXX" building i think) with dumpsters. Near one of those dumpsters you can find a cardboard. If you'll use this cardboard, you'll get sunglasses

fyi the sunglasses are a reference to the movie "they live"

Oh ok ! omw to try this out then! Thanks for the tip!