The Port of Los Santos is fitted with "A" markers where the missions Minisub, Scouting the Port and the Merryweather Heist take place and involve Trevor. Markers can also be found at the Casino where Trevor has a unique switch scene being escorted out of the Casino by security.
The Wiwang parking lot is fitted with "B" markers. Wiwang manufactures 2 way radios for taxi cabs and emergency vehicles which is solid connection to Franklin as he is the only protagonist who can purchase Downtown Cab Company. More markers can be found at USALS which means he is more than likely the protagnist who is to pick up Elisa Macallen.
The Escalera Rent-A-Car is fitted with "A" and "B" markers and the name P. Mitchell (Poppy Mitchell) who has connections to both Trevor and Franklin can be found on the bottom of the Escalera sign.