Having had this issue, and being completely let down by the main dealer.....They firstly thought it was a crack, then a casting problem..Once i had pointed it out that it was actually a drilled two mm hole, and oil was dripping, but not coming out near the oilseals, the only option offered was a completely new rear diff complete assembly.
i tried to resolve the issue, with LR customer services.
Surprise, surprise, they had no idea & referred me back to the main dealer.

As it happens, Land Rover International magazine had an article on replacing a haldex unit in that months magazine. I rang Famous Four ( whom did the article ) and they also didnt know about the 2mm hole.
The vehicle was duly booked in with them to investigate.......

The answer : When the axle breathers get clogged up, pressure builds and the excess oil / air pushes past the internal seal. This also appears to move the internal seal from its correct position.....The Hole under the internal seal is uncovered & air / oil is blown out of this hole...So it has a dual purpose : A breather hole & an indicator that the internal seal has failed....very clever volvo/ford....

So if you have this issue , speak to famous four products ( lincoln area ) , whom now have the tools & the knowledge to sort this issue....

I see, the hole is like a pressure cooker relief valve to stop the whole lot exploding, sounds like a well thought out solution i must say.

Yes - But why put a hole on the underside of a diff thats going to get wet or muddy ??

What happens if that hole clogs up, do they engineer another hole? Tongue in cheek.