Some small findings I have done, keep in mind that I haven't played GTA:SA nor seen any other webpages with GTA findings so, anyway, decided to share this.
Smoking? (Red) Small details on the NPC's, it's on the man/woman the old one. (Blue)
Mobile phones? (Green) & Someone holding a baby? (Red) LCPD, for those who haven't seen it (Blue)
Animals? Possibly a dog?
Another thing is, when it comes to the FER RY TERMINAL (or something similar) you notice "winds", perhaps the more closer to the ocean the more windy? or.. something.
edit: fixed links :p