Alarm for when patrolling units come across enemy units.

Hello guys, so seems that when patrolling units come across enemy units and begin attacking you do not hear an alarm unless the enemy unit attacks back. I feel this is not optimal. As an Arena clown, I have found that siege towers are being used more frequently. Hence, when I want to boom I tend to have one or two units patrolling the sides of my walls just in case. However, when this patrolling unit comes across a siege tower they attack it but I get no indication that it's coming.

I would like everyone else's opinion on this. Wouldn't it make sense for the devs to provide an alarm for when patrolling units come across enemy ones?