Under boot / hood inspection hatch rusty

Had cause to check my steering rack mountings the other day, trying to track down a knocking from the front. These are under inspection rubbers under the carpet that lines the boot.
Having got the carpet out and looking around, I noticed a disc about 4 1/2 inches across under the foam at the bottom. Whilst the rest of the boot floor was fine this disc was very rusty. It easily lifted up with a screwdriver under it. so some sort of inspection plate. But a mystery to what it could be inspecting. from the top all there was is the undertray. from the bottom the foam below the carpet.
Anyway after cleaning the surrounding area i was going to cut a new disc to fit but when looking around at home found a plastic disc that looked the right size. This was one of those protective discs you get in a pack of CDs. And a perfect fit! A small stopper plugged the middle hole and some sealant had it stuck in place. I took the opportunity to finish off with a quick spray paint over the area, and Bob as they say, is your uncle.
Whilst i dont use my car much in the rain it must have got wet here somehow. So something I would suggest checking whenever you have your front under tray off for any reason.
and if you can explain what on earth this disc might have been for please do let me know!
Ooops tried to load pics and think have hit a limit. pics in a bit.

 

And back with pics!
As you can see, rustier than an Italian fisherman's bucket.
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