Rear Bumper - Trapped Dirt = Corrosion?

[Gazcw]

Whilst it's possible, and aside from the usual bird s*** nothing comes to mind. I think where the plastic trim was re-fitted at some stage in it's life an opportunity has arisen for dirt/water to become trapped underneath, causing the bubbling I see on my bumper today.

I did notice that when I took the trim off, additional adhesive stickers had been applied on the underside resulting in a not-so-snug refit.

If the trim has been off before could it have had a respray and there was contamination or even a new bumper. Be interesting to see if there is another colour underneath.