Thanks Massionet51 for discovering this. 

It's in the Bahama Mamas office where Manzano's killed. It doesn't looks like any other map in GTA world. I managed to get a picture, but it has low quality and can't be readed:

Secret Bahama Mamas Map

We know that GTA V development started sometime after GTA IV was released. Considering it passed 1-2 year(s) until TBoGT was released, could this be the map of GTA V? Share your thoughts!

EDIT: I searched the web, it seems that is not Los Santos - but Massachussets, Salem more exactly.

Salem, Massachussets