Oil drip in my Escape, not sure what it is from. Rear main seal, oil pan or ???

What is your question?

Has begun dripping small amount of oil every other day about the size of a dime. Looks like coming from oil pan grate but can't fully tell. Mechanic had up on lift a month ago during oil change and states he cannot find any drip whatsoever. Drip still there. Any advice on what to do to identify what it is based on other people's experiences? Ford dealership thinks rear main seal. Car has 90K now. Thanks!