Question about an outbuilding circuit breaker box

I have a house with an outbuilding that has its own apartment and shop in it. Right now all electricity flows through the meter on the main house. There is also a meter for the outbuilding that I want to re-activate so that I can rent the apartment and the tenant can pay their own utilities.

On the panel in the outbuilding, the master switch for the panel currently doesn’t turn off the juice to it, but the breakers within that box control the various parts of the outbuilding. Is it plausible that the master switch will work once the meter is reactivated, but the electricity currently fed from the main house is coming through a breaker in the box? I need to be sure to shut off the power from the house to the outbuilding before reactivating the outbuilding meter to make sure there are not two power sources. I would give more details, but at this point I am some distance away from the house and have just been mulling over what might be going on. Thanks in advance for any help you can give.