How do aircraft differentiate between missile locks?
Non-holographic radar systems such as those used in fighter aircraft and active guided missiles only point in one direction at a time. Normally, when the radar is in search mode, a mechanism inside the radar makes the beam move from side to side (possibly up and down as well), so it has a wider field of view and can pinpoint the direction of any aircraft it finds.

The aircraft with the radar warning receiver (RWR) sees a brief pulse of radiation every second or so: only when the beam is pointing in its direction. This way, it knows the radar emitter is in search mode, and the direction of the emitter. Every aircraft in the radar's field of view will see the beam at different times, and they will all see the radar on their RWR screens. In a combat zone this is potentially a lot of emitters, but the RWR can analyse the radar signal to identify the radar: what type of aircraft, SAM, or ground radar it is, and whether it's friendly or enemy.

Getting the position of the target only every second isn't good enough to guide a missile towards it. When the radar is in track or launch mode, it stops scanning the beam, and keeps it pointed at the target aircraft.

The RWR on the target aircraft now sees much faster pulses of radiation, so it knows that it is the sole target of the radar. At this time, it can warn the pilot of a missile lock. But because the radar is now only pointing in this one direction, other aircraft in the area don't get illuminated by radar at all. Their RWR systems won't pick anything up. Only the aircraft that's actually threatened gets the missile lock warning.

In addition, the same analysis that can identify what type of radar is illuminating the aircraft can also identify fully active radar-guided missiles. Like when the aircraft's radar is in track or launch mode, the missile's radar emitter only points in one direction, and doesn't scan, so only the target aircraft will see it.



Obviously it's a little bit more complicated than this. There are a lot of details about how frequent the radar pulses are, and how the RWR knows the range of the source, but I hope I've captured all the important information in an understandable way.