You're a Manager, you're supposed to be setting an example to others. It's not OK to say you're going to turn up for a shift and then not turn up. You have more responsibility than GA's so it was probably even more important for you to be there when you were missing for that shift. I don't see any reason why you wouldn't be expecting a disciplinary for this. If you have any problems with your SM then this is a separate matter and you have options to report anything you think is not being done correctly.
I'd agree the right process is being followed. You were AWOL and some form of corrective action should be taken. A disciplinary has a lot of outcomes not necessarily a warning.Where is the desk with your personnel file? As a manager you would have access to personnel files anyway as would other managers. Being locked in a managers office or locked in a filing cabinet not sure what the difference is?
Does policy really say you must have two lets talks before any conduct disciplinary?That being said, unless this has caused serious issues or there is a history of this I would have thought a lets talk on the first occasion would be adequate.