Similar thing happened to me in a rather large country town almost 60 years ago.

I stopped in what looked like a flat parking space to attend to a service call.

I must have forgotten to apply the brakes or leave it in gear, although I didn't say that on the accident report of course.

Came out several minutes later to find the holden ute missing.

It had rolled backwards & across the two way street entering a slightly sloping alleyway that was a driveway to a small, old garage at the side of the pub.  Missed any traffic or pedestrians fortunately.

There was only minor damage to a rear bumper bar where it came in contact with an old timber pole holding up the roof.  They made the cars tough in those days & the pole did take much of the impact, bending it backwards in the process.  We pulled it back up straight again with the aid of a tow rope.