If the shliach tzibbur by mistake didn’t see there were cohanim and didn’t say the nusach of birchas cohanim and went right to “sim shalom” but really there were cohanim, what should be done now?
If the berachah of sim shalom was completed, there is nothing to do, because Birchas Kohanim is only said in the berachah of Avodah. See Kovetz Teshuvos, Vol. 3, no. 14, concerning this question.
If the berachah of sim shalom was not yet completed, the chazan should go back to the passage of the Kohanim, and they should say the berachah — like other missed parts of the davening.