Hearing a noise, the Master looked up from the very expensive desk he was sitting at as yet another secret door, one he hadn't found as of yet, opened in the office and a bald man stepped into the Master's domain.
A lesser being might have yelled, "Who the Hell are you?" But the Master wasn't a mere human with inferior memory or observational skills.
With a barely a flicker of his eyes in the direction of the large Napoleonic like painting on the wall of the man and remembering some of the documents he had found in his searching of the building the Master cocked his head to the side and said pleasantly. "Lex Luthor I presume?"