I thought that when the Chinese driver story broke recently, it was revealed that the spy was found a few years ago, not recently. So, he wouldn’t be a basis to obtain a FISA warrant in 2018, if my memory is correct. You’d have to find an agent of a foreign power to surveil who leads to DiChi.
However, there is no reason you couldn’t get a regular warrant against DiFi if there is probable cause to believe that she is engaging in illegal actions—like treason, bribery, leaking classified info, etc.
That probable cause against a sitting Senator would be a very high standard.
I think we are at the point of saying RedDriver may have had a surveillance warrant but the scope likely didn’t include the domestic discussions of the Kavanaugh issue.