My idea is basically underneath my legion where it says Guarding "....." but only your legion can see and if the planet was alerted it would say "The legion alerted planet;...." and if it wasn't alerted it would say: This planet is not (or no longer) within your scanning range.
But if the planet was down to 10 or below population the person targetting you would be able to see what your guarding (only the planet they targetted none else)
p.s: the planet it shows there is invincible so dont try find it