Having returned to the warehouse with Jenny in tow, the Master had set about ensuring that his "guest" would be unable to leave. He had used some of his hastily gathered equipment to make a rudimentary force-field capable of holding her. Then he had placed her inside, careful not to injure her. After all, he was merely using her as bait. No reason to damage the goods. He set about tinkering with what little equipment he had, all the while making notes about which components he still needed to find. He already had a plan, and now he was busily running about trying to complete it.
Indeed, the business of the moment combined with his recent capture of Jenny had put him in quite a good mood. He hummed to himself while glancing at Jenny from time to time to see if she had regained consciousness yet. After all, plans just weren't as fun without somebody to tell about them.