Fast-forward to today...
I return to the aforementioned register and tell the clerk (different clerk than the one on Friday) that I have a dress on hold. I give her my name, as well as a description of the dress, and she goes into the back area where on-hold items are to look for it. A few minutes later, she comes back...and it's not there.
"There's nothing for [my name] on the rack, and I didn't see any dress fitting the description you gave."
She invites me to go back there with her to double-check and make sure she didn't miss it, but sure enough, I do not see it. She asks me exactly what transpired on Friday, and I explain about finding the dress on the sale rack and giving it to the clerk, and so on. The next thing she says stops me cold:
"We don't do 3-day holds for sale items, only end-of-day holds."
Well, paint me red and call me Sally. I sure wish that the other clerk had told me that.
The only option left was to comb the sales racks on the floor, because presumably my dress had been put back on one of them. I was panicked because it was the only one of its kind that I'd seen, and in my size, and this was two days ago so someone could have very easily bought it. The clerk asked me again to describe it, and that's when I remembered that I took a photo of myself in the dress when I was in the fitting room, so I showed her the picture on my phone.
We come to the very end of the last sales rack, and Eureka! I spotted it, and breathed a sigh of tremendous relief. I'm very, very glad that it was still there, but really not happy with the clerk from Friday not telling me about the hold policy and the resulting shenanigans.
TL;DR: I put a dress on hold on Friday, go back to buy it today and find out it wasn't on hold, and managed to find it again after scouring all the sales racks with another clerk.