Why does my garage still have power when the house main is shut off?
To me it sounds like your true main disconnect is at the meter location. My farm has this same setup, I have a 200 amp breaker that feeds the house panel, I have a 125 amp breaker that feeds my shop panel and a 3rd that used to feed a mobile home that was here for a few years. To turn the power off to my shop I have to open the panel below my power meter to access the breakers. Some have the breakers on a side access door but this is where you should find your disconnect since turning off your main did not kill the power. Unless of course you have a second meter but that would show up on your power bill as a 2nd service.