Since your oil tank is above the oil pump, and it's been more than 7 months since it has been started, I'd guess that some oil leaked past the check valve in the pump and into the crankcase. If you don't notice this happening after short periods (days) of not running, and it's only a small amount , I wouldn't worry about it. If it is more, or happens all the time, do a search in the tips and help section for oil pump sumping on what to do about it.