It's long been the practice among security-conscious firearms owners to obscure the serial numbers in any photographs of firearms they put up on the Internet. This is because unscrupulous characters have been known to note the serial numbers, report "their" firearm stolen (with that same serial number), and claim the loss against their own insurance policies. The insurance company/ies then keep the firearm on the stolen property lists, and if there's ever an inspection (for example, you're stopped while driving, and the policeman checks your firearm serial number against his database), you may find yourself in trouble. Alternatively, if you buy a gun from a private seller and it turns out to be stolen, the same procedure may find you accused of the theft. It's happened to two people I know.
Now it emerges that Google and Facebook (and possibly others) are using optical character recognition to index the serial numbers of firearms in photographs posted or stored on their services.
Google and Facebook have now made it possible to find photos of firearms by simply typing a serial number into the search box. Earlier today, the automotive website Jalopnik published a story showing how license plate numbers are evidently scanned using optical character recognition (OCC) on Google images, allowing them to be searchable using text queries. Using the OCC hypothesis, TFB wondered if this image data mining technique might be able to be used to search for firearm serial numbers. Using images posted previously on TFB with serial numbers displayed on firearms, we tested the serial number search technique. As you can see from the results below, firearm serial numbers are in fact part of this apparent large-scale data mining operation by companies like Google and Facebook.
There's more at the link, including examples of such searches.
If you've ever put any photograph of your firearm(s) online that shows its/their serial number(s), you're at risk from this. It's not just a risk from unscrupulous characters, either. If you own a firearm for a while, and then sell it (legally, at a gun show, or something like that), and a future owner uses it to commit a crime, the serial number lookup may associate you with that crime in the eyes of law enforcement. This is not a good thing.
Forewarned is forearmed - and a useful hint not to put up any more photographs of your firearm(s) unless the serial number(s) have been obscured.