[jts1277]

The top breaker is labeled main disconnect. There is no way that is the main disconnect, right?

 

[joed]

It could be the main disconnect for the other panel inside the building. It is not the disconnect for the other breakers in that panel.
Turn it off and see what happens.

 

[tom_poconos]

If it's seated on the same metal plate as other breakers, it couldn't be a disconnect for that panel, but as joed said, it could be the feeder breaker for another subpanel.