Sounds like you've had some good advice already.
So here's the tale of my spill within a month of getting Ruby.
I spotted about a pint of water running around the boot one morning - all underneath my black plastic collapsable basket, my FIAT organiser box and my Meguiars bag of cloths and detailer. Oh no, either the rain had got in or washing her 2 days ago had done this. I threw the cloths down and soaked up as much as I could and then again after work. I had to take everything out to ensure it was all dry. It seemed ok and there was no stagnant water or any other smell - and after more rain she didn't leak again - so I was completely baffled. We'd washed her and she'd been outside in torrential rain at work before and after this incident. Nothing could account for it.
Days later I noticed my boot box had grown some white patches - oh no mould! I also noticed some white bloom all over the visible bolt heads in the boot and the screws that hold the trim in place. Mould on metal? Odd, I'm sure they weren't white before.
We'd been having a clear out at home and there were various bags and boxes piled in the hall. It was days later when I accidently kicked a bag and looked inside it and saw that it was slightly wet that realisation hit. OMG! (sorry but had to try out that phrase sometime) it's the VINEGAR!
On my first trip to BeWise (memorable for it being the first time I dared to leave Ruby in a car-park) I'd bought vinegar for cleaning. At 4% acidity it's little stronger by 1% than the stuff you put on chips, but an ideal home degreaser (windows, add a glug to washing-up water) and gentle descaler. It was a bargain so I bought 4x750ml bottles and popped them in the boot in a plastic carrier bag. Got home, took carrier bag indoors, then left it in a corner for a week, til we got round to organising a kitchen cupboard the following weekend.
I never once suspected it was vinegar. There was no smell in my boot whatsoever, it was just like mopping up water.
it seems the acidic-ness just made the bolts and screws go a funny colour but all's well. Hubby cleaned them up with some light oil (3-in-1 type) and it appears to be all surface only and lifted off easily so restoring the metal surface.
I did buy another boot box organiser but there's one thing which I did have in the boot to mention as this saved the day - I have the boot liner tray. It covers the floor carpeting and also has a lip around it. So what vinegar didn't evaporate from the initial spill was therefore completely contained in the tray for me to find that morning.
I was quite anguished at first thinking my hard-won dream machine was leaky, then the thought of her
rusting away in the acid fumes was horrible - but thankfully all's well. So if you haven't got one, I can personally recommend having the boot tray liner for damage limitation purposes, in your 500 too!
http://www.fiat-accessories.com/Fiat-500-Boot-Liner
(maybe a FF trader can rustle up a group discount if enough interest from members is shown in this item even??)