I think this is normal. There is a small load on the battery from the clock. Although the current is small. When you connect the voltmeter from the disconnected negative cable to the negative battery terminal, you are completing the circuit for the clock and possibly other loads such as the dome light if the dorrs are open. The voltmeter reads the difference in voltage between the negative terminal and the positive terminal through the clock circuitry. This should be close to 12v.