The technique is called a "canary trap" or "barium meal test": https://en.wikipedia.org/wiki/Canary_trap https://en.wikipedia.org/wiki/Canary_trap No idea if a

The technique is called a "canary trap" or "barium meal test": https://en.wikipedia.org/wiki/Canary_trap https://en.wikipedia.org/wiki/Canary_trap No idea if anyone offers it as a service. It's unlikely because the primary consumer of the service would be someone trying to prevent documents from leaking, so trusting them to the service is itself a major point of failure.