i work on cars for a living. in almost 30 years, i've never left a car up in the air without double-checking the safety locks.
on inground lifts, i generally put a deadman under them in addition to the lock on the anti-rotator bar. on above ground lifts like the one pictured, i always lower the car onto the locks. if it looks like it's still dropping on one side, then i know they;re not engaged.
i only ever leave cars in the air when absolutely necessary though. seems there was a large bit of carelessness there to me.