You constantly have to render people in when they can’t be seen but will soon be seen. Which also means instead of keeping track of just locations the server needs to render the scene in sufficient detail as to determine sightlines.
Usually games just do this by sending info to clients of where everyone is and letting the clients render people in when the client determines that the sightline isn’t interrupted.
Some games will just not send the positions until they’re within a certain range of each other, but I’m a realistic game like tark you’d need several kilometers of info in case someone scoped in.
If you don’t do this correctly it leads to characters popping into existence from thin air