An oil line or oil passage that is open, broken or disconnected will cause low engine oil pressure. The engine builder should check each oil passage for debris and wear. Sometimes oil galleries can be out of alignment where the holes are not lined up correctly to allow normal oil flow. The second choice is to drop the oil pan and reach in and get the item. Depending on the car, this may be the most direct and efficient way to address the problem. Order a new pan gasket, drain the oil, undo the two dozen or so little bolts holding the pan to the block, pull out the wayward nut (and any other little nut friends you find in there as well), clean it up, put it back together, done. An oil soaked rag under the hood is a great way to light your car on fire. By putting tape around the rag and using the rag as an impromtu filter, the air pressure of the crank case vacuum will prevent the oil from saturating the rag. Get a new oil cap immediately. Then get an oil change.