Atm one has constantly check the value to notice a change.
If you want to script things like events or behavior change based on awareness level, this is quite costly - hence a EH hook would be great.
Depending on the underlying engine code different approaches may be the most meaningful [in regards to EH design and parameters]:
+ per unit and all its knownTargets
+ per unit and a specific target unit
+ mission EH for any unit about its knownTargets