Sounds to me like an app set to run on boot would be the cause for that toast message. Do you happen to have the Device Unlock app on your phone? I'm assuming you have phone variant for a specific network/carrier. I know that for T-Mobile's Device Unlock app, there are two options to unlock device: Permanent Unlock, and Temporary Unlock. If you recently met your carrier's requirements for unlocking your device, the actual unlock may have been done automatically on your behalf. ORRRR, someone close to your phone (or maybe it was you if say you're one to take sleeping meds occasionally) tinkered with the app at some point in the past. All speculation at this point on the WHEN or HOW, but the WHY and WHAT all point to that one app.