Orange markers labeled A and B can be found on light posts across Los Santos and Blaine County. What is interesting about the markers is that they're posted in locations that involve Trevor and Franklin, take a look.
A - Trevor (Missions and Stunt Jumps) Note: Nigel calls Trevor Jock Cranley who is a stunt double.
Multiple stunt jumps can be be found at The Port of Los Santos and Merryweather Pier. Both locations are also where the missions Minisub, Scouting the Port and the Merryweather Heist take place.
Note: The marker may also represent the smart approach during the Merryweather Heist.
The above ground parking on South Boulevard is a stunt jump.
Trevor is escorted out of the Lucky 7 Casino by security during a unique switch scene.
The marker found at Globe Oil gas station in Paleto Bay could mean that Trevor is in charge of blowing up gas stations because a can of gas can be collected from the pumps. The building is fitted with a CCTV camera but cannot be purchased.
B Franklin (Missions and Cab Fares)
The markers can be found in the car lot of Wiwang who manufactures 2 way radios for taxi cabs and Franklin is the only protagonist who can purchase Downtown Cab Company.
I believe the markers found on the USALS lamp posts point to Elisa Macallen who is a special cab fare and the only mission found in the immediate area.
A - B
As of now a stunt jump and monkey mosaic can be found at The Secure Unit in Banning but there may be more to this location, possibly another cab fare.
Poppy Mitchell's name can be found on the information sign at Escalara Car Rental. Franklin is involved in the paparazzo missions "The Sex Tape" and "Meltdown" where Franklin and Michael are given a choice to kill Poppy during her arrest but would be the wrong decision to make as she makes appearances in both the Doomsday and Nightlife DLC's. Franklin assassinates the owner of Vapid with the help of Lester. Trevor can purchase the Vapid garage in Pillbox Hill which has a stunt jump on the east end of the property.
The Vespucci Beach parking lot doesn't see much action when it comes to collectibles, stunt jumps or missions involving Trevor or Franklin but both characters do have unique switch scenes spawning in or near the parking lot but without solid evidence there may be more to discover in the area, possibly another cab fare.
There could be more of these markers hidden around the map that I have not discovered, if you come across one please add its location in the comments, thanks.