The three groups of monsters didn't believe for a second that their actions were being hidden. Since they believed the forest was aiding the demon, they also believed that the forest was aware of their gathering and had already informed him.
What they didn't know was that Pandora wasn't aware of them at all, her reach didn't go past the boundary. But that didn't mean Noah was unaware. Since he was prepared for something like this, he had already made plans.
Above the trees, in inconspicuous locations around the edges of the entire wall, little slimes could be spotted. They were looking across the wall, acting as lookouts. These slimes were uniquely similar to Ailetta's. However, their eyes didn't have the same sentimental appeal as hers. They were merely puppets being manually controlled by Noah's will.
