A quick clean down of the area where the oil is seen and above with a good engine de-greaser and a hose down with care, then leave the undertray off for a week or so with the occasional glance to see where it fist appears may give a clue as to where it is coming from.
When, if, it does appear, you will then be able to follow the leak upwards to the source.
Easier said than done I'm afraid in a crowded engine bay, the use of inspection mirrors, good lights and an inspection camera may help, but it can be a guestimate if the area of the leak isn't accessible.
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Common sense isn't a gift, it's a punishment because you have to deal with everyone who doesn't have it. 
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