It helps to watch match replays from within the sensor view. Then you can see what the radars are illuminating and what the seekers are looking at.
The lock box indicates what the radar is tracking, however the radar “beam” for lack of a better term is still quite large and anything within it will generate a return that a SARH missile will potentially track. Think of SARHs like IR missiles but the “heat” is the radar reflection. Anything in you radar zone will generate “heat” and the missile will go for the “hottest” target.
Well to be fair infrared is a form of radiation
yeah, thats what I was getting at, IR is generated by the target while radar is generated by your plane and reflected back by the target, in either case if another plane gets close to the target the missile will go for what ever has the stronger signature, not what they where initially locked on to. so you do need to treat both types of missiles in a similar manner in regards to friendlies, but at least with SARH you can turn the radar off if they go for friendlies.