Yesterday I was fitting new number plates to my VRS when I noticed some rust bubbling around the number plate lights. After removing the lights and having a scrape I found the left side to be quite bad. Out with the rust treatment, etch primer, white primer and top coat and all is well now. Was a bit suprised as the rest of the car has no rust (that I can see)

Welcome.

Sadly very common across different Skoda models for a good few years now. 

Galvanic corrosion happens a lot in that area 

I thought the lamp holder unit was plastic?

 

Looks more like a capillary water trap combined with the naturally thin electrostatic zinc primer coat on panel edges to me.

 

Even if the housing was a dissimilar metal the fixings go through rubber insulating grommets so I am giving a myth busted for galvanic corrosion in this instance.

If you're doing a general look for rust, it's worth having a look at the inner arch lips (especially if the foam blocks are still behind the arch liners), and the backs of the inner sills.

 

My 06 has just started to get a little surface rust on the inner sills. Not bad for 16 years / 200k. 

9 hours ago, J.R. said:

 

 

Looks more like a capillary water trap combined with the naturally thin electrostatic zinc primer coat on panel edges to me.

 

 

This👍