Came across a few of these suckers... unidentified oil leak coming from the front cover of a 6.7L PowerStroke in a 2013. Covers front of oil pan with wet oil. Hard to pinpoint the location. Using florescent oil dyes, we made an assumption that it could be the front main seal. We started a tear down to determine the root cause and devise an approach to repair it.
The process of removal of all accessory drive components is somewhat of a hassle. I wrote an article covering the replacement of the water pump. Please follow those directions until step 8. Upon removing the cooling fan drive assembly, we located the source of the oil leak to be the vacuum pump seal. While attempting to replace the seal, we noticed the bolts were loose causing the leak - we replaced the seal anyway.
In retrospect, tightening the bolts could have rectified the situation. The amount of labor required to repeat the job if the seal did need replaced easily justifies the cost of a new seal upfront so it's better to be safe than sorry. The preventative tightening of the bolts anytime the front accessories have been disassembled could prevent future leaks on other vehicles. Perhaps a dab of locktite on the bolts could prevent repeat repairs.
Update: 05/14/2016: I have done several of these by now. Gets easier with repetition, but everything does. On one vehicle, I had to replace the radiator - if that's something your currently doing it would be wise to go ahead and remove what is needed to check these bolts.