If you attack an enemy from a non-hidden tile, and then move onto a hidden tile with your remaining APs, that enemy can see you through the bushes and is able (e.g.) to fire bows at you.
It is possible that they hit your hidden bro inadvertently while aiming at an adjacent bro, thus revealing your location. I did the exact same thing to the A.I., and revealed a hidden bandit adjacent to my intended target.
No. The hidden one was flanking the enemy in isolation.