I watched a TV show recently that told of a Brit who was fundamental in code breaking after the Enigma was captured.
He developed a technique called Traffic Analysis to place all the radio signals in context and derive intelligence there from. His work remained secret for a long time, even after the divulgence of the enigma posession. The technique is fundamental to the intelligence business today. It is still oficially secret.
Obama being commander in chief commanded the resources and tecnique to be used against his domestic enemies, namely Donald Trump. We have not yet leaned of the others that we can be certain who were surveiled...... all of the Republican candidates at a minimum
Not exactly secret!
https://en.wikipedia.org/wiki/Traffic_analysis
and marketing analysts used an advanced version (with AI - Machine & Deep Learning algorithms) today. Its really a subset of social network analysis
https://en.wikipedia.org/wiki/Social_network_analysis