It is common to find pin holes in oil pans of engines that have set for long periods of time with old sludgy oil. Drops of water sink to the bottom and become very acid from the old oil. The sludge is thick enough that the water stays in droplets and eats it's way into the pan. If you look closely when you clean out the pan there will be perfectly round pits eaten into the steel.
Well cleaned and washed in lacquer thinner and a coat of J B weld (not the fast set) over the area does nicely. Don't just do the spots that leak because there will be more that are only paper thin.