All electrical circuits must be grounded. Some permanently some with virtual ground. The virtual ground has voltage against the permanent ground as in virtual ground for high voltage circuits in hybrid and electric vehicles. Even airplanes when it returns to earth must be grounded to dissipate static electricity it has accumulated when it was in flight before passengers and crew disembark or before aircraft maintainers touch the plane. The grounded circuit in question is probably from a circuit system that has the switch on the ground side which in this case automotive negative ground. This problem does not blow a fuse but leaves the circuit load powered up unintentionally without being noticed. All electrical circuits must be grounded for safety, for reliability of signals of sensors, for electromagnetic interference (EMI), for radio frequency interference (RFI) and for cancellation of unwanted static electricity