Why did Mr. Can Daan accuse cart of blackmail?

This is because the man knew of a door leading to the loft. He wanted money to keep quiet.

Here is the text,

Mr. Kraler. That’s the man. A couple of weeks ago, when I was in the storeroom, he closed the door and asked me . . . “How’s Mr. Frank? What do you hear from Mr. Frank?” I told him I only knew there was a rumor that you were in Switzerland. He said he’d heard that rumor too, but he thought I might know something more. I didn’t pay any attention to it . . . but then a thing happened yesterday . . . He’d brought some invoices to the office for me to sign. As I was going through them, I looked up. He was standing staring at the bookcase . . . your bookcase. He said he thought he remembered a door there . . . Wasn’t there a door there that used to go up to the loft? Then he told me he wanted more money. Twenty guilders more a week.

Mr. Van Daan. Blackmail!

Mr. Frank. Twenty guilders? Very modest blackmail.