A Power Probe will read any circuit that's not currently hooked up to power, but is still hooked up to ground, as ground, even if there's some small resistance in the circuit (e.g., lightbulbs). That does not mean the circuit is grounded out!
#electrical
Make sure to check logic boards for error codes in the form of flashing LED patterns or clicking/buzzing noises...it can save a lot of time. This thought brought to you by spending way too long figuring out what why a garage door opener wouldn't go down.
If a car new enough to have an anti-theft system isn’t starting, check that the anti-theft system is working as expected before doing any further diagnostics. Could just be that the key is dead.