Paragon is a word used to reffer to someone that goes excellent in life, or it is really good on what they do. Also it can described a characteristic of them that highlights on them because it is way better than the normal amount of it. For example: Their way of studying is a paragon of organization, they are never behind with the reading.
It would have warned the criminals to be on guard. He also said he didn't want Watson to accidentally lead to his discovery by coming to see him or bringing him things.