If I understand this story correctly, you have quite a bit of run time on it since the oil change and then the oil drain plug just decided to fall out after you parked it at the barn door?
I surmise that because you still have the drain plug to add new oil and/or notice that the bolt gasket is cracked/uneven, so you must have found it where the tractor was parked.
On the other hand if it fell out while you were mowing, the tractor was likely run without oil for a lot more than the 20 feet/1 minute you are suggesting.
I wonder
