Saab 9-3 linear 2009 - spare well full of water?

I'd noticed my boot liner was getting a tad mouldy. I assumed it was due to the gym bag I haven't used in around 6 months (don't judge me on use of gym or duration bag has been in boot).

As I live in England, and we have sun for about 5 minutes a year, I haven't checked this out as it's been raining for about 7 months.

I opened up the boot to dry it out today, lifted up to the spare and there's about 5 inches of water.

Cannot for the life of me find where its coming from, as all fabric is dry, purely a puddle in the well.

Any thoughts or similar experience??