What does Palmer find written on the icing of his cake later in the evening of his birthday? Who does he believe wrote it? What does he think it means?

Palmer goes into the kitchen to eat a slice of cake and finds written in the icing the work "tonight." After thinking about it, he believes Henry has left the warning for him. He believes Beans and Mutto plan to enter his room and find Nipper, perhaps try and wring his neck.