yup same thing
noticed this on mine after a week long sub-zero temps. I need to find the link for it, but there is a ford service bulletin out on this. Essentially, in vehicles kept in extreme cold conditions, there appears to be an initial leak of assembly lube outside of the seals. This is confirmed through the use of UV dye added to the rear end and then checked with a UV light. In most cases, it was found there was no leak of the axle seal into the differential. Therefore, the area is cleaned with solvent and returned to the customer. No further leaks were detected.
Yeah, I know, this sounds crazy at first, but, my vehicle spent most of its life in the SW part of the US and had never been subjected to sub-zero temps. When I bought the vehicle in the spring of 2015, there was not a drip of fluid, rust or anything under it. I inspected it with a fine tooth comb. When this happened there were 3 small drips on the garage floor below the differential. I did an internet search on the issue and found that service bulletin. As luck would have it, I did clean the area with solvent, and never had another drip afterward. Call me crazy, but it was confirmed by the dealer who performed a dye test and found no evidence of a leak. I had the gear lube and gasket changed as a precaution and, again, no leaks.
I will try to re-locate that service bulletin and post it here if I can find it.
