There are a couple of ways you can proceed in this situation, one of which is to physically destroy the thumbdrives with a pair of pliers. I mean, perhaps your Mom used it to bring a copy of her taxes to the accountant, or you have spreadsheets of invoices payable from your clients back when you weren’t using the Cloud?
You’re smart to make sure that the USB Flashdrives you’re finding (or are they “Flash Drives” two words? I never remember!) are empty of data because you just never know what might be on it and identity theft isn’t just in the movies, unfortunately.