This is a classic case of a collection agent being unable to verify / validate a debt. This is a common occurrence. Here, the collection agent happens to be an attorney, but the law applies equally to all who try to collect a debt.
To understand the basics of verifying or validating a debt (the terms are synonymous) see the Bills.com resource Debt Validation.
Under the Fair Debt Collection Practices Act (FDCPA), if a creditor cannot verify a debt it may not collect the debt or otherwise contact the debtor about the debt. If the collection agent does not contact you further about the debt, it is safe to assume your collection account was a bare account, and therefore is non-collectable.
Should the present owner of your collection account sell or assign the account to another collection agent, you may be contacted again. Like before, validate the debt. If the account contains no documentation, that collection agent may not collect the debt, contact you about the debt, or report it to the credit reporting agencies.
I hope this information helps you Find. Learn & Save.