You bought the car after it had its last oil change. How long after you bought till you took it to the dealer for the inspection? I would say a safety inspection doesnt involve any use of rags, furthermore I would guess that if they jammed a rag in the oil tank that it would be pretty hard for it to make it to the oil pan...Im thinking almost impossible.
You can see that if they jammed the rag in the tank that it would have to travel through a hose to make it to the oil pan which looks almost impossibe on its own, then who knows what kind of filter is at the bottom of that tank to stop foreign debris from going to the pan.
My guess is that work was done to your motor. The mechanic wont be able to tell as all the gaskets are metal and rubber with no rtv applied anywhere. Especially on a new motor. But I would go to the guy you bought it from. Probably a reason he was selling it. Its too coincidential to die that fast.