This dissonance seems to come from the fact that citing has two purposes

    1. Assigning credit to where you heard about something.
    2. Giving a claim (the best possible, and sufficiently strong) support.

From the credit perspective you should cite wherever you heard something, even if it was alleyway graffiti. From the support perspective, you should take the idea you read on the wall look it up to see if someone credible has said the same thing, and then cite them, and if not maybe not mention it all.