There are other scenario's, the patient could have been on COBRA and the carrier had not updated allowing time for the premium to come in..when it didn't it is allowed to retroactive the term date, another situation could be due to EOI investigations to which the patient did not disclose a condition on their EOI and the coverage was "rescinded"
There's also a slight possibility the claim was paid to the insured because the carrier missed the AOB and then they reprocessed and requested refund from patient.
I wouldn't worry about it for now unless they come to you for payment back.