Every time a foreign agent solicits services from a US politician, as well as those of our allies, there are anywhere from 3-24 different surveillance methods monitoring what the conversation is, where the parties are located, where they were before and after the contact, whether action was taken in responding to the approach, etc. Bank accounts, consulates, airports, and any physical and financial chokepoints are monitored by surveillance people, who are given BOLOs for the specific actors.
This kind of stuff pre-dates the digital world as well, when it was largely HUMINT, IMINT (Image Intelligence), and SIGINT (Signals Intelligence). For example, the US Intel Community had extremely high priority assets placed on monitoring the Soviet nuclear triad forces, and went to lengths that would blow your mind to be able to see, know, and act before they could.
That was so detailed already in the 1950s and 1960s, that we knew every freaking bomber pilot, his wife, the co-pilots, flight engineers, bombardier-navigators, maintenance personnel, their bases, phone networks were all tapped, their alert code words, and a specific targeting cell within each of the US services had technicians and briefers working 24/7 on shifts on a giant plotting board where everyone and everything was shown for senior planners.
The US Navy tracked Soviet submarines with their SLBMs as one example. The Navy invested billions in putting satellites into space to track everything possible related to that, from Murmansk to the White Sea, Arctic Ocean, North Atlantic, etc. Any imagery from the satellites was relayed to the ASW platforms covering the region, including Norwegians and USN ASW aircraft.
CIA and NSA saw all of this and expanded their scope, and the Soviets used their assets in the CIA to try to work around it, but US technical surveillance was so ominous as to be basically impossible to counter. The only way to really negate it is to control the WH and DoD as much as possible, as well as fill Congress with as many moles as possible.
This is why genuine patriots at the lower levels within the IC get blocked all the time from pursuing the natural course of their surveillance on compromised politicians.