Just to close this out, the unicom traffic on the KROC feed is coming from Penn Yan (KPEO), about 40nm away from Rochester. I still don't know whether the pilots are using the wrong frequency, or the scanner is picking up something extra. But because the controllers don't correct the pilots, if they are on the ROC frequency by mistake, they must be far enough away that ROC doesn't pick them up (but the feed receiver does? odd).