Quite often, while roaming around as Franklin and thinking I should be doing something more responsible than playing a game, if I see a car or truck that I don't remember customizing before, I'll steal it and drive up to Franklin's shop to trick it out with free goodies. Often it's the same old stuff - wheels, paint, armor, whatever. But of course, some cars let you install cool stuff like Mad Max-style superchargers sticking up through the hood, huge wings on the trunk, curb feelers, carbon parts, side pipes, etc.
As Franklin, I tricked out a dull Bobcat pickup truck with a 1980s beige & brown two-tone paint job with steel wheels and chrome dog-dish hubcaps and, the most important thing - white letter tires!
It was still slow.
Somewhat bored and aimless, I drove over to Trevor's house, parked the truck, saved the game on Franklin's phone, and then switched players to see what Trevor was up to. He spawned in his bedroom, as if he had just gotten out of bed after saving the game there. I walked outside as Trevor and figured I'd go on another police helicopter killing spree from a flatbed/lowboy train car. I ran to the train tracks near Trevor's house and hopped on the train that, as usual, was conveniently heading my way as soon as I approached the tracks.
I jumped on and pulled out my Combat MG to look around for convenient targets on the road that runs next to the tracks in Sandy Shores. I was surprised to see not one but two of the custom trucks I had just painted and installed white-letter tires on. That was really strange. Usually when I switch characters, the vehicle the previous character was in is gone forever (unless it's their personal ones that always return home or to wherever they spawn).
Near the stop sign where you turn to get from Trevor's house to the airport, the darts place, and Franklin's garage, I (as Trevor) jumped off the train to see about those trucks. As I approached them, they stopped and Franklin appeared on the street - not me as Franklin, but Franklin as an NPC, I guess. I was still playing as Trevor. Franklin asked if Trevor wanted to hang out and I was given a choice to accept or decline. I accepted and the darts place lit up in yellow on my map as a destination. I got into the first of the two duplicate, custom trucks as Trevor. Franklin got in the passenger side and they had a conversation about Michael and other events they've been through as I drove to the darts place.
They played darts and afterwards Franklin asked if Trevor wanted to go somewhere else or drop him off. A waypoint was set to a run-down liquor store in Sandy Shores. As I drove there, I noticed a bunch of places all over the map with martini glasses highlighted in yellow - bars or restaurants, I guess. I hadn't seen that before that I can remember. I drove to the waypoint and Franklin got out and said something like, "let's do this again sometime" before walking away. Maybe I should have gone to one of the bars to see what happens next. I certainly will if I get a similar opportunity again.
I then, still as Trevor, drove the truck Franklin had customized over to Franklin's shop and saw it still had the Stage 4 modifications for everything that Franklin installed - engine, armor, etc. This is the first time I've been able to swap a customized vehicle from one player to another in the game. I don't think this is important, but it is different.