Rear diff heat shield adrift...?

[Zip]

Whilst under my car removing the rear bumper to replace a dicky parking sensor, noticed a "damp" rear diff. Not sure where the leak is from at the moment, but more intriguingly, adrift and wedged behind it was what can only be described as an office sandwich platter sized and shaped piece of aluminium with four points of dramatic corrosion around the edge that looked distinctly like it should have been bolted somewhere...

It remained wedged during 2 170 mile journeys this weekend.

Anything to be concerned about? I assume it gets hot under there...

Where should it be fixed? Not sure that it looks very replaceable without droppin the rear diff...!!

Zip