Go to this location on the map

First enter inside, then you will see a staircase leading to the upper floor.



After that, go to the balcony of the building through this door on the upper floor.



Then jump onto the metal roof on your left.



After walking all the way to the end of the roof, jump to the part in front of the roof.



Then go up to the other part of the roof.



After moving forward on the roof you will find the hidden note.

On the note it says :

If you like my guide, don't forget to add it to your favorites. Also, if you have enough steam points, you can give an award to my guide. This fact motivates me a lot anyway, good games.

Also you can find the secret note by watching this video :