Yeah, that's right, I forgot about Stett's "remote" thermostat over on the right side of the engine compartment. I'd check those fittings that go from the thermostat to the cooler itself. If it's a slow ooze, it's likely a fitting. Even a pinhole leak in the hose would cause a lot of oil to drip down onto the underpan and eventually onto the floor.
2013 F30 335i M-Sport 8AT/MPE/MPPK, 2016 228i M-Sport Cabrio 8AT