Can't Find Main Panel Breaker
That's not a split bus (Rule of Six). For one thing, it is too new - Rule of Six panels were outlawed before CH panels started getting branded Eaton.    For another, all the breakers are accounted for, there would need to be a  "Main Lighting" breaker. 

By the way, CH is a great panel.   

I wonder if the main panel used to be here, and is now on the other side of the wall, and this one was installed as a subpanel to handle the dozen or so circuits that were impracticable to route to the new location.