There are multiple sub panels as the house has had wings added over time by previous owners.
The one problem with my "test" of hitting one of the breakers is that breaker could, I guess, have actually been going to a sub panel, in which case all circuits on that sub panel would have gone out.
I think someone said it doesn't really matter if it's on a sub panel anyway because the white wire is physically wired as a ground in the main panel and in the sub panels, not as a neutral.
The branch circuit has to originate at the service (250.140). I believe that has always been the requirement. If it goes to a subpanel, it violated the NEC when installed.
At a subpanel, if you are connecting a drier and connect the white wire to "ground" you are putting current on the ground wires. Among the problems is if the ground back to the source fails this could make all the downstream grounds hot. If you connect to the neutral bar, if there is a problem with the supply neutral the drier frame could be way above ground potential.
IMHO you should at least determine what the major breakers feed and label them.
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