Which of the following is NOT a visual clue indicating a package may be dangerous?

The correct answer highlights that a typed return address label is not typically considered a visual clue indicating that a package may be dangerous. In standard shipping and mailing practices, a typed label is a common and acceptable method for addressing packages. It suggests professionalism and legibility, which is often seen in business correspondence.

In contrast, the other options serve as potential indicators of risk. Excessive securing materials, such as string or masking tape, may suggest that someone is trying to disguise the contents or ensure that they don't escape in transit, which could raise suspicions. A package labeled "CONFIDENTIAL for Mr. Smith" might indicate a more sensitive nature of the contents, which can be a red flag for potential tampering or misuse, especially in certain contexts. Oily stains, discolorations, or crystallization are physical manifestations that could indicate chemical hazards or other dangerous materials within the package. Each of these clues points to irregularities that warrant further investigation, while a typed return address label does not inherently suggest any danger.