If a non-Jew is present in the synagogue, such as if maintenance workers come during Shaharit to make repairs, or the custodian comes in to take a chair, may Birkat Kohanim be recited, or must the Kohanim wait until the non-Jew leaves?
The Poskim address this question in the context of the Shulhan Aruch’s ruling (Orah Haim 55) regarding a case where people hear Kaddish or Kedusha, or a Beracha, but there is a non-Jew situated in between them and the Hazan. The Shulhan Aruch rules in such a case that those who hear the recitation cannot respond, as the non-Jew constitutes an "interruption" between them and the one who recited the Beracha. One might, initially, conclude on this basis that the presence of a non-Jew "interrupts" the Kohanim’s blessing, and thus the blessing should not be recited while the non-Jew is in the synagogue. In truth, however, this is incorrect. The Hida (Rav Haim Yosef David Azulai, 1724-1806), in Yosef Ometz (70), writes that the Shulhan Aruch speaks only of a case where the Beracha is recited in one room, and people hear the recitation in a different room. In such a case, the people in the different room cannot respond unless there is no separation between them and the person who recited the Beracha. But if everyone is in the same room, the presence of a gentile does not constitute any sort of interruption. As such, it is entirely permissible for Birkat Kohanim to be recited when a gentile is present. This ruling appears in the work Orech Yamim (p. 326; listen to audio recording for precise citation).
Summary: If a non-Jew enters or is present in the synagogue during Birkat Kohanim, the Kohanim may recite the blessing as usual, and they do not have to wait until the non-Jew leaves.