On November 11 of last year, mom was mailed a bill for $1,154.67 from a company called Cavalry Portfolio Services. That company seems to have purchased a debt mom owed to HSBC Bank Nevada, which was almost certainly a Visa or Mastercard that she had stopped paying.
I called this morning and the woman who answered the phone asked for the case number on the bill. Once I gave it to her, I explained to her that mom had died on August 12th of this year and that I was (slowly and methodically) trying to deal with the stacks of paperwork and bills my sister found in her apartment. The woman said she was terribly sorry for our loss and that she would update the account to reflect that mom was deceased and there was nothing more I needed to do.