Imagine it was the time of the Cold War between America and Russia. Americans are trying to find out what the Russians' plans are for armaments. They plan to put a spy and are now selecting candidates for the job. All candidates pass the foundation training: learn Russian language, Russian culture, Russian music, Russian history. After all the tests, they choose one candidate who is the best in everything. The day comes when the selected candidate gets a Russian passport and goes to Russia. At the airport in Moscow, a customs officer takes his passport, looks at it and
says "Sir, I'm arresting you"
(You need to give an answer on the basis of how the Russians recognized him as a spy?