code/datums/components/proximity_monitor.dm

/datum/component/proximity_monitor	Attaching this component to an atom means that the atom will be able to detect mobs or objects moving within a specified radius of it.
/obj/effect/abstract/proximity_checker	Inteded for use with the proximity checker component /datum/component/proximity_monitor. Whenever a movable atom crosses this object, it calls `HasProximity()` on the object which is listening for proximity (`hasprox_receiver`).