Filler Bung fell out off rear axel

Hi, this happened to me.
I fixed a leak off the rear axel, I discovered and cleaned the blocked rear breather banjo bolt.
I topped up the oil all was fine.

Then I drove to work a few days later, I hear a bang from the back (just thought it was a tool sliding). At work I see some funny oil stains over the back door (Thought someone sprayed some cleaner near my car).

When I left work the next morning I reversed out of the space and heard a metal Bang! Like I ran something over. Pulled forward and found nothing behind so I drove home.

Now a few days later I've discovered the filler bung is missing from the axel and there's no oil in there. I drove about 10-15miles with no oil in it.

My question is How much damage have I caused?

Any advice would be great please.
Many thanks
Gray

Only way to find out is pop the diff off. If the filler plug is inside then it may have caused some damage.

Good news is that if it is a 90 then a 2nd hand diff can be got fairly cheaply.

Not a difficult job , but be aware the diff is a heavy piece of kit to move around.

Cheers