It's either in "The Art of Intelligence", or "The Great War of Our Time where the author states that every country will place a spy in certain diplomatic positions. He mentions that the CIA places case officers in certain Secretary of State positions at certain embassies/consulates.
But that Russia is notorious and much more aggresive for doing such. So much so that if you are talking to a Russian diplomat or Ambassador you can almost be certain he/she is a spy.
Heck, if you watch the documentary Icarus about Russian doping, they mention agents from the FSB security service positioned at sites around Sochi to facilitate there doping program.