So I've got the map mostly to myself except for 2 blips, a green one in Fort Mercer, and the other blue one at El Presidio. The guy at Fort Mercer continually restarts the hideout (no doubt farming XP), but the guy at El Presidio doesn't seem to be doing anything.
I watch the map for a bit when a third, red blip comes in also at El Presidio. I watch the red blip approach the blue, and the blue vanishes. It pops up again as blue re-spawns nearby and red moves to him and again the blue blip blinks out. Red approaches blue after the respawn, and then quits the session.
Curious, I went to take a look, and found blue was literally just standing around El Presidio. No one seemed to be in control of the character at all. So I used him to get the horse trampling achievement. All the way to level 3... :D
I have no idea why someone would start multiplayer and then walk away. I sent the person a friend request just to see if they respond. It would suck to find out they'd died holding the controller.
But as I was checking the stats I realized I still needed the Lemming achievement, and, being at El Presidio already, began jumping off the cliff behind the fort. I ran to the same spot and repeatedly flung my character off.
Most of the time, the game would re-spawn me pretty close to the spot I was jumping, and when the camera would spiral out of the death scene, I began to notice that there were multiple corpses at the bottom of he cliff. And every single one was my character. Sometimes there were as many as ten of me: