There are two scenarios in which a Hidden Characters Alert can appear.
First, the alert can signify that characters have been added to the text and hidden from the naked eye in white "ink." Commonly, a thin letter such as an "I" will be inserted into the spaces of a text and then hidden in a white text color to deceive a plagiarism scan. Typically, other plagiarism detectors do not pick up on these text segments as long chunks of original content exist and exclude them from being detected as plagiarized content.
Second, the alert can appear because text images are strategically inserted into the document. In this case, a student would format an image with text to appear as though it were not an image but rather a part of the written text of the document. Typically, other plagiarism detectors do not detect the image as text and simply ignore it, excluding it from being detected as plagiarized content.
Below is a video with an example of the alert from this sample report.