Unfortunately it's not an uncommon problem this time of year, rain followed by a bit of sun usually guarantees a bit of condensation.
Belive it or not, the acrylic used for the window is not impervious to water and will let microscopic water droplets through into the cavity.
The easiest solution is to do nothing, wait for a dry day and let the sun shine on the window, more often than not this will solve the problem. Failing that you could remove the small bungs in the window and gently warm the acrylic with a hairdryer and then replace the bungs (assuming you haven't desroyed them whilst removing them, that is!).
If this fails to cure it then you are probably looking at a window that has delaminated and will need repairing/replacing.
Have a look around the forums and you will see that you are not alone with this problem!
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