He recommended recording serial numbers, microdot markers or serial number recording programmes.
From the nzherald.co.nz
The embryologist places the prepared sperm on the microdot, selects and captures sperm that bind to the dot.
From the en.wikipedia.org
I'd like to thank blotter, windowpane, Owlsley white, mescaline, shrooms, purple microdot and all of their cousins.
From the swampland.time.com
The ticket is printed over a microdot pattern readable by the pen's camera, so it knows which box the officer is checking off.
From the sltrib.com
Showing it to Luther in private, they discover a magnetic microdot under the stamp, which Luther takes to his contacts to decode.
From the en.wikipedia.org
What is really amazing is that an insect with a microdot-sized brain can handle this type of image analysis when we have entire regions of brain dedicated to the problem.
From the sciencedaily.com
The second project Raskar demonstrated was a system called Bokodes, a microdot that is invisible when its in focus but is clear when you defocus the surface.
From the techcrunch.com
The stolen documents are examined, which includes a photograph of the cargo ship Ning-Po and a microdot message saying the tourist who took the photo was killed as a security precaution.
From the en.wikipedia.org
More examples
Photograph reduced to the size of a dot (usually for purposes of security)
A microdot is text or an image substantially reduced in size onto a 1mm disc to prevent detection by unintended recipients. Microdots are normally circular around one millimetre in diameter but can be made into different shapes and sizes and made from various materials such as polyester. ...
The Microdot is a concept design by William Towns for a small, economical town car. The car was first shown in 1976 and was an evolution of his 1972 Minissima car. The Microdot was a petrol/electric hybrid vehicle, designed to carry three people side-by-side on short city journeys.
Microdot, a company previously based in South Pasadena, California and now owned by Tyco Electronics manufactures a wide range of coaxial and multi-pin connectors that have been used since the 1950's for instrumentation and aerospace applications. ...
A photographic image that has been reduced in size to that of a dot (typically around 1mm in size and circular) in order to escape detection. ^[1]
A photographic reduction of a secret message so small it can be hidden in plain sight under the period at the end of this sentence.
A miniature picture the size of the head of a pin.