Thought i would post this up as i came across it at work today, had a 2009 octavia with the cooling fan coming on when engine turned off and ran until it flattened the battery, after lots of head scratching, it turned out to be water in the engine fuse box, stripped the fuse box and found all the contacts green. from the outside it looked fine but upon splitting it, it was toast.

Skoda do the part and was only a hundred quid or so , easy fix for an anoying fault,

hope this helps some one !!!!