I honestly don't know the legalities, from a DND perspective, of skipping that last leg of a flight. I know the airlines hate it (it causes them paperwork and can cost them money - Lufthansa recently sued a guy for skipping the last leg of his flight to get a lower fare (look up 'hidden city' flights or skiplagging)). And if you do this on the outbound leg, the return ticket will be cancelled by the airline!!!! I haven't come across this in any Travel Directives I've read through. Maybe it is addressed somewhere; I just haven't seen it.
Anyhow I can see both sides of this one. As a former CO, my first concern would be whether you were ok. If I got a call from my Clerk saying you weren't on your return flight, me or the RSM or your company leadership would be dropping everything to try and find you, even after hours. We might even call the MPs to help ensure you are safe. That level and type of attention from your command group is not something you should be happy about! Once we knew you were safe, my next question to the Clerk would be: will this cause the Unit any problems with future bookings and will it cost the Unit any money?
So, your actions are not just about yourself. The impacts can be broader. Can be, but may not be (I really can't see how this could actually cost DND any extra money).
The preferred way would have been, as suggested above, to coordinate it ahead of time. I reckon you ack that point by now. Another option, and always a good idea when you are straying from a plan, would have been to call your immediate supervisor to let him know what you were doing. He may have looked into it with the OR and come back with a thumbs up. Or, he may have made a decision on the spot, and said OK. In that case, you can rightly say you asked and were given verbal authority (although that puts your boss on the spot, but oh well, he made the call). I can certainly visualize a phone call between more senior Members doing this - a Captain calling his OC or a WO calling his CSM to let them know they're starting their leave a few hours early and a few hundred KM away from Base, and there being no consequences. But, those guys will have been around long enough to know what's reasonable and what is not. But they'd likely, and wisely, make that quick phone call, rather than just disappear off the face of the Earth.
There's something to be said for acting first then seeking permission later, but that usually doesn't play so well when the benefit is personal, rather than operational.
Personally I hope they let you off with nothing. You come across as a decent guy and this isn't really so bad an 'infraction'. Whatever happens, take your lumps and hopefully your Unit isn't led and administered by asses.
