In 1998, one of our branches found a couple pieces of jewelry left behind in a customer safe deposit room. They stored these items in one of the bank safe deposit boxes without any note as to whom it may belong, just that they were found in the room during inspection. Today, branch staff found these items after discovering the bank box keys (this is a box that has not been used in over 15 years).

What is the proper way of handling these items when we do not know which customer may have lost them?

i think you need to follow your state guidelines for escheatment

At the time the items were found, your bank should have identified the last time the booth was inspected and found to be empty, then contacted each renter who has used the booth in the interim and asked them to come in and verify the contents of their box.

As that was not done in a timely fashion, the property should have been escheated years ago...

I would definitely add to your safe box procedures that after each customer visits, a visual inspection should be made that the room is empty/nothing was left behind. We do this with the customer and simply say "let's just make sure nothing was left behind". I can't tell you how many cusotmers have thanked us for this "going the extra mile" and it eliminates problems like the one above.