Take the sop a bit further; Door shall only be open or closed by ground staff.
That way, the stairs/steps/bridge must be in place anytime the door is moved. Subtle, but very effective.
If it was a non-revenue flight, it pays to refer to some u.s. research that indicates a third of fatalities on all part 121 operations occur on non-revenue operations.
The point being, we tend to let our guard down when its not a scheduled flight..