It’s a mistake, certainly (though it could easily have been propped open only for a short time). You need to decide what you want. Is it a grovelling apology or firm action taken against the staff member. Or is it something else? You need to communicate this to the manager and see where that gets you. Personally, I would have a face to face discussion with the manager, impress on him that this (obviously) shouldn’t happen and might require retraining, and leave it at that.