I had been out observing for a couple of hours earlier in the evening and was just having a final look around. So far as I can tell, the skies were perfectly clear. But I suppose a passing puff of cloud might be responsible. Or some other obstruction of course.
What surprised me was that Megrez was not simply faint but invisible, at least to the naked eye. That’s something I have never seen (or not seen) before in over 60 years of skywatching.
I will be having a closer look tonight with optical aid.