"I notice you don't have all of them." She leaned toward the bars and gazed out at the empty cells. Before she stepped back and returned to him. "Not strong enough?" She wasn't trying to taunt him. Simply point out that his experiment was lacking something key.
"But to answer your question, all I heard was metal scraping against metal for a while, and then a click, and the squeak of the hinges when the door was opened. That's all I heard." She knew the metal against metal could be anything, but she figured it was just someone picking a lock. "Does that tell you anything?"