I should have said before, don't label up the panels that way until you verify that is what's happening. Really need to get the dead fronts off and take some pics. Going to bed to trace the supply wires feeding each panel to make sure of its source.
To clarify RI Chevy's bootleg comment: the picture he posted is how it is supposed to be done. "Bootlegged" is incorrect or stolen power and should never be done.
Not saying you are- just an in general statement:
I don't care if someone is doing it to steal power form the power company and feeding half their building from pre meter, it's a dangerous method. I would never encourage anyone to steal, but if people do it they need to at least do it safely. A fine or a couple months in jail is one thing- people dying is another.
I have family members that like to keep hidden their employment by the power company to avoid issues. But I frequently hear of "this house burned down" or "the electrocution of that person". Most of the time it's the resident.
Back when I tired leaving wrench turning for a while and became an electrician, I did many re-wire of houses. I've seen enough aftermath of what seems like a safe thing to most people. I've also seen things that make you wonder how people can breathe by themselves they are so stupid.
Sorry for the rant, I know I get worked up sometimes...