I had a customer a few years back who was doing the same thing with pennies. Although, instead of giving us an assortment of weird stories, he told us up front that he was sorting the pennies in order to extract the older ones that have a high copper content. He even posted videos on facebook that demonstrated his penny sorting machine.
It's entirely possible that your customer is doing something similar in search of silver. And since some of the more enthusiastic silver hoarders don't want a whole lot of competition, it wouldn't surprise me to hear someone provide an alternate story, lest the bank staff be inclined to searh the dimes themselves.
That he doesn't bring the coins back is also consistent with this type of activity. If he's sifting the dimes, then he's probably returning the ones that he isn't keeping to another branch, or even another bank, in order to avoid sifting the same pile over and over. This was my experience with my penny customer. He had a few "pickup" branches and a separate "dropoff" branch.
Depending on where in PA you're located, your customer may even know my customer...