A document is usually thought of as its contents: the words on the page, the figures in the table, the image in the frame. But a document produced and shared in the ordinary modern way carries more than its visible contents. Beneath the text sits a layer of information about the document itself, recorded automatically, and that layer can be as identifying as anything on the page.

This attached information varies by the kind of file, but the pattern is familiar. A document may carry the name under which a piece of software was registered, the identity of the account that created or last edited it, the dates of its making and revision, and sometimes the device or location involved. A photograph may carry the settings of the camera and, in some cases, the place it was taken. None of this is part of the visible content, and most people who create such files are not thinking about it.

The consequence is that a document can disclose its own origin. A file shared on the understanding that it is anonymous, or that it came from one source rather than another, may quietly contradict that understanding through the information attached beneath it. The text says one thing; the layer beneath it says who made it, and when, and with what.

This matters in two directions. It means that documents a person sends out may be carrying more than the person intends, identifying them or their circumstances to a recipient who thinks to look. And it means that documents a person receives can be examined in the same way, which is a capability worth knowing exists, whether one is the sender or the subject.

The information is not malicious and not hidden in any sinister sense. It is recorded for practical reasons, to help software manage files, and it is simply not displayed. But not displayed is not the same as not present, and a person careful about their exposure should know that the documents passing through their hands are rarely as silent about their origins as they appear.

The practical discipline is modest. It is to be aware that files carry this layer, to understand which kinds carry what, and to attend to it before a document leaves one's control, particularly a document whose value depends on discretion about where it came from. A document is a more talkative object than it looks, and the part that talks most plainly is the part one cannot see.