Long term storage or sitting the converter can drain back. There is a drainback valve to stop that. If overnight it drains back, that valve is likely not functioning correctly. Not running the oil level should be high by an inch or so. When started cold it should be a bit low but in the operating range. If cold and it is even close to the low mark, go for a 3 to 5 mile drive to get the oil up to temperature, then check and top up if required.