There is a misconception that since in the synagogue the Kohanim go in front of the Aron for Birkat Kohanim, the Beracha may be recited only in the presence of a Sefer Torah. The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in Parashat Tesave (13), writes that this is incorrect (listen to audio recording for precise citation). Even if a Minyan prays in a house or other place without a Sefer Torah, the Kohanim stand in the front of the room and recite Birkat Kohanim.
If a person who is not participating in the Minyan happens to enter the synagogue during Birkat Kohanim, such as if he came to get his Tallit and Tefillin for a later Minyan, or happens to be passing through, he should stand silently in place until after Birkat Kohanim. Even though he does not pray in that Minyan, he should remain in the synagogue through the end of Birkat Kohanim.
If a Kohen had already prayed and recited Birkat Kohanim in an earlier Minyan, and, for whatever reason, he finds himself in a different Minyan at the time of Birkat Kohanim, he may recite Birkat Kohanim again at that Minyan. There is no limit on how many times a Kohen may recite Birkat Kohanim on any given day; anytime he finds himself in a Minyan at the time of Birkat Kohanim, he may recite the Beracha, even if he had already done so earlier in the day.
Summary: Birkat Kohanim is recited even in a place without a Sefer Torah. One who happens to be in a synagogue during Birkat Kohanim must stand in place silently and listen to the Beracha, even if he prayed at an earlier Minyan or will pray at a later Minyan. A Kohen who had already prayed and recited Birkat Kohanim may recite the Beracha again if he happens to be at another Minyan at the time of Birkat Kohanim. There is no limit on how many times a Kohen may recite Birkat Kohanim on any given day.