What is the point of having scouts if they're not secretive?
If all they do is shout at the enemy they are coming, then scamper off without actually fighting, then instead of building scouts, there should be an option to scout with any unit.
If you wanted to make it more interesting, then you could have an element of 'luck' or 'skill' when scouting to see if they are discovered.
but basically...if the person you are scouting has no scouts to detect your scouts, then the 'defender' shouldn't know about it.