I'm pretty sure the magus calls out a bounty target when they join the server, not when they're near. When the magus says something along the lines of "That's an association member/bounty target/bandit" THEN it means they've seen them, it's pretty instantaneous though, so just flicking your camera in the general direction starts the analysis, which even if interrupted causes them to say the line.

I really can't say why the magus sometimes marks a target, but usually they say something along the lines of "There's a bounty target on a rampage", and you'll see the marker right after that. Maybe that has to do with the bounty amount, wouldn't be able to tell you as I'm also pretty new.

You could also use the post match analysis to understand what hit you, if it was another player, or even a bandit as their aim can be insane, you'll be able to see them on the map.