Pin 16 on the ODB-II connector is specified as "Battery Power" or "Battery Positive". There will always be 12v power between pin 4 and 16. The connector will always have power on these two pins regardless of the key position, and when you plug a device into the port it will have power and may come on by itself (many scanners/tuners do this). This can drain some power from the vehicle's battery.
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