Art, artifacts, collectibles and other valuables are often security marked in case of theft, loss, dispute or other later need to identify the item and/or owners. The markers range from overt holograms and serial numbered stickers to invisible tags, and allow the marked items, and often the rightful owners, to be identified.
An interesting covert marking system uses microdots. Microdots are microscopically small metal discs that have identifying information microetched on them. The information can be read under a microscope. The dots can be the size of standard printed period (.). The etched information can be a unique serial number that identifies the object’s owner. Applied via clear adhesive to a valuable painting or sculpture, the dots will go completely unnoticed by the average thief, but can be used to trace the item back to the rightful owner. The adhesive contains dye that can be seen only under black light, so that the area where the microdots are can be found.
Microdots and related covert ‘shrinking down text’ is an old-time application. German spies used microdots to covertly pass information during WWII. Even as early as the 1870 Franco-Prussian War, carrier pigeon messages were photographically shrunk so the bird could carry more text.
Microdots are commercially available and can be purchased by the average collector. I've even seen them for sale on eBay.