Why does Carl’s make behavior make Mr. Kraler nervous?

Carl seems to know that Mr. Frank is not in Switzerland as rumored. He also acts suspiciously in the office and asks for more money....... a bit of blackmail.

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.