In his letter to Sherlock Holmes in "The Adventure of the Sussex Vampire," Robert Ferguson claims that he is writing on behalf of someone else. Watson is taken in by Ferguson's subterfuge, but Holmes guesses straight away that the case is related to Ferguson himself. Evidently, Ferguson feels somewhat embarrassed at relating a vampire story to one of England's finest minds in case he should be thought to have taken leave of his senses. As we discover later, he also wants to protect his wife's good name while at the same time protecting his children. That being the case, he cannot go to the police.

Holmes replies to Ferguson's letter by telegram. We don't know exactly what Holmes writes in it, but when Ferguson turns up the following day in Holmes's study he makes it abundantly clear that Holmes has seen through his little charade:

I see by your telegram, Mr. Holmes, that it is no use my pretending to be anyone's deputy.