Check your Asterisk Phonebook for an entry, it could be in there. Without knowing what you are pointing your CID lookup to, which is within your inbound route for the DID, it is hard to say.
CID can be passed from the trunk provider if the service is offered, if you have it turned off and you are still getting a name, it is cached in your DB somewhere, was you using CID SuperFecta by any chance at any point?