The base layer consists of sensors on the platform itself that perceive a threat: Missile Warning Systems (MWS) detect the infrared plume of an incoming missile, Radar Warning Receivers (RWR) detect a radar lock, and laser warning detects a directed laser.
The gap
None of these sensors see a cold, non-targeting threat: a drone that places no radar lock, or a transiting cruise missile not aimed at the own aircraft. This is the open gap that additional radar/EO detection of small targets addresses — a development not yet fully mature anywhere.
See the reference card MWS / RWR.