I was in a freeplay lobby and noticed a player standing still. I tentatively went up the woman who just stood there for ages with a black Adder parked in the middle of the road behind her. After a while she turned around and started walking as if she was a pedestrian and not a player.
It was almost like the player logged out and the character became a random ped.
Your avatar remains in the world for a while after you leave, you can even create multiple duplicates of yourself if you have a friend watch over the mindless npc you become when you leave.
It's actually quite creepy how you can watch a fully functional living thinking avatar disconnect and turn into something the computer is controlling. Makes the whole "Matrix" feel of GTAO a pretty good mind f*ck. Or I smoke too much weed...