This all seems so implausible.
When you say you left the car and the doors locked automatically, did you verify by pulling a door handle to make sure they were locked? How are you certain they locked? Could the car have chirped to let you know that you shut the door and walked away while it was still running?
The way many of these systems work (FCA, BMW, etc) is that if you attempt to put the car in Park and fail to do so, it reverts to Neutral. This is what killed the young Star Trek actor Anton Yelchin.
Yelchin drove his Grand Cherokee to the top of his sloped driveway and with a shift mechanism similar to the Bolt, he thought he engaged Park. However the car was still either slightly moving while he tried to engage Park (imperceptible) or he failed to fully push the lever towards Park and the system put the car in Neutral. There is no visual or tactile cue that an FCA shifter like this has engaged a gear (much like the Bolt). He then walked down his driveway to get the mail and his Jeep was slowly and silently rolling backwards down the grade without him knowing. The Jeep rolled into him and crushed him against the mailbox. FCA has subsequently changed the Grand Cherokee shifter to a conventional PRNDL arrangement.
Can you try parking your car in the garage, putting it in Neutral (don't switch off the car), shutting the door, and walking away? My hypothesis here is that you didn't fully engage Park, forgot to shut off the car's power switch, shut the door, and then you walked away. The car chirped the horn to alert you of the problem and you assumed it was chirping to tell you the doors were locked. Like the Volt, after a period of time, the Bolt might shut off the ignition by itself if left in a running state unoccupied. The car would now be powered down and in Neutral, allowing for either slight movement or other outside influence, all the while you assumed the door was locked and the car was in Park.
Just a wild-ass guess.