With a "real" warrant, I will eventually find out that it's been issued, either because I'll be served with it, or because the evidence will show up in court and the prosecutor will have to explain how they got it. It's totally possible that literally all people on Earth are currently being surveilled under a single FISA warrant, with any prosecutions in real courts being disguised by "parallel construction". We don't, can't, and probably won't ever know if this is the case.