hi guys just had a look round the GSA and found a slight oil/grease leak from around the drive shaft boot on the drive box end not a lot but should the be cause for concern
But you've answered your own question HERE , check the levels in the rear bevel and gearbox, if one is low, thats where the leak is. If the levels are ok, it's the seal breathing, don't worry about it, just keep your eye on the levels every now and again.
It's in every service schedule " check the oil levels"