"player_joined <id>" fires on all scripts the first time a player enters the world.
"game_playerjoin <id>" fires also fires whenever a player joins the server, but I think it only applies to \sv_world
"ext_activate_items <id>" fires on all scripts whenever a player respawns (and when he first joins in world.)
"bs_global_command <id> vanish death" also fires on all scripts whenever a player dies. (Check PARAM3 for 'death', as this same event also fires whenever a player uses a global summon command, such as "all follow" or "all vanish".)
Should probably be added to the stx at least, as soft/hard events. Really need a wiki or something for the scripting documentation, which some stabs were made at, but yeah, epic task.
Don't think we have a reliable way to passively detect disconnects. game_playerleave fires in sv_world, which in turn fires player_left globally, but it doesn't seem to go off under every circumstance.