I knew about metadata but this was new to me

The different sensitivities of the photosites creates a type of imperceptible image watermark. Although unintentional, it acts like a fingerprint, unique to your camera’s sensor, which is imprinted onto every photo you take. Much like snowflakes, no two imaging sensors are alike.

In the digital image forensics community, this sensor fingerprint is known as "photo response non-uniformity". And it's "difficult to remove even when one tries", says Jessica Fridrich of Binghamton University in New York state. It's inherent to the sensor, as opposed to measures, such as photo metadata, that are "intentionally implemented", she explains.

This is good to know too. I always suspected that printers had something like this.

When considering these privacy issues, we might draw parallels with another technology. Many colour printers add secret tracking dots to documents: virtually invisible yellow dots that reveal a printer's serial number, as well as the date and time a document was printed