we were doing some ground testing around the car- still chasing down Coyote issues...

my son used a multimeter between the battery ground and the alternator case - it measured ~15v ?!
we tested the wires to the alternator and the pin read 12v from battery

we disconnected the battery, even disconnected our extra fuse panel, tripped the 150A breaker between the alternator and the battery - and eventually the 'charge' went away

we started the car a few more times, but never saw that 'charge' on the alternator case again

what could cause such a thing to happen?