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Oil drain is on the oil pan on the bottom of the engine. The usual culprits are a leaking oil pan or even rust through or a leaking rear seal. If you look over the back of the engine you should see if it is the rear seal. A small rust hole can be anywhere in the pan. A bright light and mirror is the only way to find the leak without a dye in the oil and a UV light.
What engine do you have?
Try this. Get some butchers paper or large sheet of white craft paper. Make sure the bilge is clean and dry. Put the paper under the engine. Run the engine on a flushing devise. Run until the engine warms up. Shut down the engine. Remove the paper and see where the oil drips are. It should pinpoint where the leak is.
Post back what you find.
Check carefully back by the flywheel housing and cover. Your seal can leak into the housing and you won't see a leak.