Me’ilah 27 - Use of Priestly Garments (Tamid)
In addition to the Kohanim guarding the Temple Courtyard from the outside (because they were forbidden to sit inside), there were twenty-one guards of the Levites guarding the Temple at the gates and other places, from the inside.
To return to the previous subject , the Kohanim did not sleep in their garments but rolled them under their heads. Do we see from here that only sleeping in them was prohibited, but daytime use, such as walking in them, was in fact allowed, even when not doing the service? - No, this is no proof. Perhaps daytime use was also prohibited, but the teacher chose to speak about sleep, to tell us that the garments should be treated respectfully. If so, how could they put the garments under their heads? - Don’t read "under" but read "next to" their heads.
But consider the story of Alexander the Macedonian marching to destroy the Temple, at the incitement of the Cutheans. What did Shimon the Righteous, High Priest, do? He donned the priestly garments and went to meet Alexander. They were travelling all night, and at dawn, when Alexander saw the High Priest, he alighted from his horse and bowed down to him. They asked Alexander, "Why are you bowing down to this Jew?" He replied, "I see this image every time I go into battle, and it protects me." Don’t we see from this story that priestly garments can be used for personal needs? - No, we don’t, perhaps they were garments fit for service, but not actual priestly garments. Or perhaps they were the real garments, but "Time to act for God" is an exception.
The conclusion is that priestly garments can be used, but then there is further disagreement whether it means using them even before the service, or only not taking them off after the service.
Art: Sebastiano Conca - Alexander the Great in the Temple of Jerusalem