Hello, My name is XXXXX XXXXX I will help with your question. Let me first ask, are you speaking of the oil pan drain plug? The plug you remove to drain the engine oil? thanks.
OK thanks Eclayton
One of my hipothesis is that, due to the scratchs in the oil pan, a stone impacted the plug (hexagonal head type), produced a tangencial force couterclockwise that unscrew the plug and then with vibration it falls...
Does this make sense for you, according to your experience?
I'll rate immediatly after your next reply.