Ceramic hob, reason for discolouration?

Hi, I'm having my first encounter with a ceramic hob. Although I find it easy to keep the surface of the hob clean and shiny, there's a streaky discolouration *under* and glass on a couple of the burners after use.
This seems to come and go a bit, and moves around at every use. I'm careful to only use the rings with pans on them, etc. etc....
I'm interested to find out what is causing this? Any ideas?
Thnx. Barb
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