They were in one of Caleb's hideouts, and no one except maybe Kelan knew about this place. It was some sort of warehouse where things that humans used to work the fields were stored, but from the layer of dust on the materials, this place must have been abandoned long ago.
Solomon really had no idea what this place was, but when he saw Ian open a trapdoor hidden by a complex seal, his curiosity grew.
He knew that this kind of seal was used to protect hideouts, but he had never seen one this complex. Ian opened it easily, though, and they went down into a basement where there was a small armory with very good quality weapons, a large meeting room, and several cells that were all empty except for the farthest one, where there was a woman inside.
Solomon could have recognized her anywhere because he had once considered her his own sister and he really thought that he knew her by heart... but the reality have been really cruel.