Pro Tip: Click here to return an item using the Returns and Refunds feature. Amazon doesn't check if the items that get returned have all their parts in the package. That's one of the security reasons behind your 20% restocking fee.
Yes and no, Amazon checks returns to make sure that the items are new and in good condition. If they're damaged then the return gets an 'unfulfilled' status. However, Amazon's staff don't check to see if the item had any extras with it. It's possible for sellers to get a returned product that's missing a few parts. It's an unfortunate event, and this security issue is one of the reasons for the restocking fee.
Amazon.com requires customers to pay 20% of the original purchase price on a returned item. It's usually able to help cover issues like missing parts.