In act 2, scene 1, Mr. Kraler asks to discuss something with Mr. Frank alone. Rather than keep the matter secret, Mr. Frank has Mr. Kraler tell everyone what is going on. He reveals that a workman from the warehouse asked about the Franks' well-being a couple of weeks ago. Then yesterday the man approached Mr. Kraler again and asked about the bookcase and if there weren't stairs leading to a loft behind it. Then he asked for twenty guilders more a week. Based on the workman's behavior, Mr. Van Daan is first to say what everyone else is thinking, "Blackmail!" Dussel responds with the following:
"You know what I think? He was the thief who was down there that night. That's how he knows we're here."
Mr. Dussel's belief seems reasonable. Mr. Kraler asks Mr. Frank what he should do about the matter. Mr. Frank decides that they should offer the man half--ten guilders--and then see if it really is blackmail or not.
Considering the fact that Peter accidentally dropped a lamp on the night that the burglar was in the warehouse, it is possible that the workman figured that Jews could be hiding upstairs. Then, the workman probably figured out that Mr. Frank would be the most logical one to be in hiding above his former workplace. Due to the desperate nature of life during wartime, the workman must have felt as though he was in a position to demand blackmail money, so he does.