Q. A taalis brocho question: At... a makeshift shul... , including a chazzan's amud on wheels that has an opening door... I kept my (new) Shabbos Taallis inside of the amud zipped shut in a standard Taallis bag with my name sewn on it. I came one Shabbos and my Taallis was gone and another was left in its place, albeit inferior to my new and taller one. Can I make a brocho on the one left in my bag? Can I keep it? This wasn't merely a matter of people confusing or swapping taallesim somewhere. I did not take my Taallis out as I was sick that week and not in shul. Someone went into the amud's interior space, went into my bag, unzipped it, took out my Taalis and then put a different Taallis back.
A. Horav Shlomo Miller's Shlit'a opinion is that you have to be meffarsem and announce publicly to the people of the minyan your issue at hand. Although, because of the movable nature of the minyan you may not be able to place a sign (maybe on the movable amud), still during the minyan on Shabbos it should be announced. After advertising for at least three weeks, since the individual who hopefully by mistake, took your taallis owes you the taallis or a repayment for it, you may collect the one left in its place and recite a brocho.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a