When you take a photo, your smartphone or digital camera stores "metadata" within the image file. This automatically and parasitically burrows itself into every photo you take. It is data about data, providing identifying information such as when and where an image was captured, and what type of camera was used.

This lack of awareness has proven useful for police investigators, to help them place unwitting criminals at a scene. But it also poses a privacy problem for law-abiding citizens if the authorities can track their activities through images on their camera and social media. And unfortunately, savvy criminals can use the same tricks as the police: if they can discover where and when a photo was taken, it can leave you vulnerable to crimes such as burglary or stalking.

But metadata is not the only thing hidden in your photos. There is also a unique personal identifier linking every image you capture to the specific camera used, but it's one you'd probably never suspect. Even professional photographers might not realise or remember that it's there.

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.