What are the odds that since case is high profile they probably used some grey area or illegal surveillance tool from their 3 letter agency friends and made up some story how a patron recognized him.

Convinently guy also had a copy of all the incriminatint stuff with him. (In russia he would also have sims3).

Since he talked on phone, they used some voice recognition software. They probably record all the calls and the system recognized him when he made a new one (or even worse - from someone elses' phone).

Then they used this whole doctrine of not revealing the real source.