Sometimes it's written "Du" even if in the middle of the sentence when addressing someone directly. It's technically incorrect, but it's used for emphasis and hence politeness, and that's probably where your feeling comes from.
The same can happen with other words that are getting capitalized for similar reasons, but when going strictly by the book it's grammatically incorrect. An example would be "das Große Ganze" where it should be "große" but it is capitalized to emphasize the connection/phrase.