It doesn't take much oil at all to collect back there. All it takes is a few drops getting exposure to the air going under the body as the car is in motion. Might not be enough to have drips either.
If it was from oily exhaust from the engine, it'd be putting out a smoke cloud (i.e. fogging mosquitoes!) that was noticeable AND might get you reported to the air quality board in some states. Not to mention fouling out spark plugs, I suspect.
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