One thing that's good about losing something in a shop is that it's usually a closed system, nothing gets out unchecked except for garbage. So if you lost it in the shop at least you know it's probably still in the shop...somewhere.
I’m not sure that the shop can really be called a closed system in
these days of athletic-type shoes. Take a look at the bottom of a
modern shoe - they often feature a series of narrow rubber grooves
that are perfect for picking up and concealing a small stone,
without the wearer having any knowledge of it. If you still haven’t
found that diamond, and someone in the shop wears sneakers, you might
make a close inspection of those sneaky things…