What do I need to know about the death of a loved one and their Social Security check?

You don't need to do anything. Social Security will handle the reimbursement via the bank themselves.... and the bank will send out a notice to the address being used by your Uncle saying that Social Security took back the check.

Social Security pays a month behind, so the check that was received Nov 2nd is for the month of October. There is nothing for you to do other than notifying SS of your uncles passing, if you haven't already done so.

There's a difference between SSI and Social Security and they are paid on different days of the month.

If he was paid on the first he was getting SSI and I believe that is payment for the month it is issued, not a month behind, like SS.