Note that you don't actually have to see the flare itself to know it's there. A prime example is sunrise and sunset - light remains for quite a while even after the direct rays of light are gone. Same principle applies to all light sources.
Hmm, wonder if we can make this any more complex....
Just a thought, rules for sound detection might be similar too - at least both have a "burst radius" and are effected in similar ways by obstacles between the source and detection point.
We got any mathmeticians here to help out?