Noticed this the other night after I came in from work late. It was a very cold evening and just before closing the garage door I noticed my Zed had begun to facelift itself...
I'm off to check again now and see if it's swapped the bumper over yet.
Easy fixed though - new back covers from a stealers, £25. Old ones off, hair dryer for 10 mins on hot, short blow on a cool setting to get rid of moist air, fit new back cover, back to ginger in no time
The rubber seals on the covers perish and let moisture in after a few years. The fogs are a different story on the facelift though, they are just crap :x
It's only done this the once on a particularly cold night. Hasn't done it since. They are generally clear of any condensation and I put new back covers on a few months ago as a precaution.
I have factory Xenons and LED sidelights both of which run cooler than halogens. On the night I saw the condensation I think I had used my main beam for a while on the back roads so the halogen bulbs may have put a lot of heat into the lamp on a cold night thus causing a wee bit of condensation. I don't think it necessarily means I have a leak or standing water in there but who knows.
Cool... I'd keep an eye on it though as eventually mine had quite a bit of water in them as more and more condensation formed, then dripped down and collected in the bottom.