Well, that is a new one. A repeat inspection for O2 quantity checking? I am not talking about daily/weekly, but more of an out of phase or MEL requirement. A bit iffy that?! I can imagine why though. We were reminiscing last night, as you do?! and I recalled the time when, as an apprentice, we spent all week on a B747, trying to get the pax oxy ring main to stop leaking. It is a big system.
If you were fault finding, you would imagine the fault to be within the cabin or flight deck, because this is the most probable place of a system disturbance. If you use an oxy charging point, as it appears QANTAS do, you would imagine that the last place the leak could be, would be on the bottle. If it had been changed recently, you would check it, but if not, you would be looking at flight deck masks, or passenger O2 PSU's with a bottle of snoop in your hand.
I still can't work out how the bottle let go though. The regulator valve yes, but bits of bottle?