Exactly. Once they
know exactly what someone has done and how, it's relatively easy for them to find alternative means of "suspecting" that person of doing the crime and convince the judge to give them a warrant for exactly what they've already found through the illegal surveillance operation.
I wish judges and defense attorneys would catch on to these tactics more quickly. The rate at which the prosecutors/FBI invent new tricks to fool the courts and defense attorneys so far seems to far outpace the judge and the defense attorneys' understanding of what's even happening.
Take cell site simulators, for instance - the FBI has used those in secret for more than a decade before they were uncovered at all, and then it took another decade for judges here and there to catch-up and start requiring warrants for such operations.
And this goes for a lot of FBI's "investigative techniques", too, which are often illegal, but what judge is really going to know the difference between those highly technical operations?