In reality, government operatives from a host of three-letter agencies are working to develop large networks of informants. These are mostly folks who deal with other people and are in the know– the bartender in Beirut, the luxury car dealer in Bogota, the money changer in Riyadh, the hotel manager in Shanghai, etc.
These assets are constantly being pumped for information– who did you see, what were they buying, where did they go next, who were they with, what were they discussing, etc. And in exchange, informants typically get paid.
In the United States, there are a number of laws on the books which are theoretically supposed to prevent the three letter agencies from spying on US citizens. Naturally, the government dispenses with such inconvenient formalities in its sole discretion, and Congress frequently passes legislative exceptions (USA PATRIOT Act, NDAA, etc.)
There’s a little known division of the Treasury Department called the Financial Crimes Enforcement Network (FinCEN) whose mission is to “to enhance U.S. national security, deter and detect criminal activity, and safeguard financial systems from abuse by promoting transparency in the U.S. and international financial systems.”