I had a mystery leak once.

Any leak above the lowest point will appear to be leaking from that lowest point.
A friend suggested an old trick.

Clean everything with Brake Cleaner and a rag. Especially around the places mentioned previously in this thread. Spray everywhere with some WD40 and blow baby powder on it. I used a blow nozzle set at about 25psi and just blew it out of my hand. You can be creative on this. Once you have a good coating of powder on the case, pan and possible leak points, let it dry for an hour then drive a couple miles and re-inspect.

My oil leak was was dripping from the bell housing and I thought it was a rear seal. It turned out to be the oil pressure sender on top the block. I swear I never saw any trails of oil before, but I certainly did with the powder.