“Halachically speaking, if you are in shul and don’t have tefillin for whatever reason, and no one is available to lend you a pair for whatever reason, it is mutar to use a pair that you find, provided you handle them gently and put them back exactly as you found them. This, for once, does not come from my husband– I just read it in an ArtScroll book about the halachos of traveling.”
This is a halacha in Shulchan Aruch, in Hilchos Tefillin. I’m glad it’s in the Artscroll book too.
The reason for this halacha is because we assume that a person would want a mitzvah to be performed with his belongings.
The Mishna Berurah points out that this does not apply, under any circumstances, to sefarim or siddurim, because there is a big chance that they will get damaged while being used. (Interestingly, he adds that sefarim are assur even for casual use, because we are choshesh that the person using them may get caught up in a sugya and use the sefer “heavily”, causing it to rip.)