Best answer by JAQ-PC
The only thing I can think of is if you blocked one your factory installed apps from mobile data & it's stuck somehow. It might be one of the one's that you can't disable completely, like from your mobile provider or something. In your settings check the "Data" or "Data Usage" menu. That's all I can think of at the moment.
EDIT- One more thought: if you hold down on the notification (like you were going to block it or change it priority) it should say what app it's for. Maybe you can pinpoint your problem app that way. Worth a shot anyhow