This can happen if there is a change in the way the account is reported by the lender or creditor.
For instance, if a bank reports an account number to Equifax as 12345678 one day, Equifax credit monitoring will look for that account number the following day.
If the bank changes the way it reports the account number -- if it appears as 01234567, for example -- Equifax may send you an alert saying the old account has been closed and a new account has been opened.