I think (although the websites don't say) that a queen wasp might come out of hibernation and start to build then either die because the weather has been so terrible, or find that the spot that she "thought" (do wasps think?) was a nice quiet one is in fact a busy one due to gardeners coming out of hibernation too. I often find part built nests in and around my sheds and garage, but very rarely see a completed nest. The advice from the website is to keep an eye on it to see if it continues to be built or not. If I found one at that stage somewhere that I need to be or need the wasps not to be, then i would remove it and hope that the wasp finds a better place to build.