[mortov]

It is standard practice in classified material communities to have carefully controlled variants of documents and a log of where and when the subtle variations go in order to detect and source leaks of information. When any particular area is detected as suspect, misinformation dressed up to look more valuable is then carefully spread in that direction to see who takes the bait. They are then either used to send misinformation or, if they have served their purpose or have nothing to offer, removed (fired/imprisoned or whatever is appropriate).

The Americans (TV Series) had a couple of themes based around this - it's not exactly a secret strategy and receivers of information know to verify what they get before getting too excited.

Since we're talking about government actors here, nowadays they would not be easily fooled and would have skilled engineers checking stuff no matter what, so that is not going to add any 'cost' to the operation. People in the espionage game learn pretty quickly or they don't get to play for very long.