It is of course possible that if there is a meter box on the granny flat as you’ve described, that it’s a sub-meter being used to record the granny-flat’s usage, and which is then being deducted from the main meter’s usage before bills are issued.
It may be less confrontational and direct to just approach the landlord and say you’ve observed that your breakers cut off their power, and you’re concerned about the bills – and could they show you their bills to assuage your concerns.
Also, if you’re in Vic, I think granny-flats aren’t allowed to be separately metered – they’re ‘dependent dwellings’ – but it’s been a while since I checked that.