Each and everytime I've encountered this situation, I've been told (by AC staff) that it's AC's responsibility to have an AC agent there to unlock the doors. Furthermore, when this has happened, there has never been a government official (i.e. Customs) present.

Given that the entire area is secure and that you can only make your way into the customs hall, I see no reason why this would be insufficient. So I think it fair to say that it was AC's responsibility. After all, it's not up to customs for the GTAA to meet and greet flights at the gate.

And I'm pretty sure the plane didn't magically re-appear at the gate without anyone at AC operations knowing about it.