Update, fyi. Vet says he loves the condition Sara is in - he used the word excellent. Blood tests came back negative, no problems, no physical issues. I just posted a photo of the two dogs in the pet pics thread, but I've also added the photo below.
Given that we don't find anything physical wrong with her, I'm thinking she is probably just getting old enough she doesn't want to get up in the middle of the night so much any more, and if her leg (the bad knee) is also bothering her, then she is reluctant to go upstairs. Put all that together, and I think that is the k.i.s.s. answer!
Cheers and thanks!
Oh, yeah - Sara is the hound in the foreground, smiling for the camera. Klinger is the Lab/G. Shorthair pointer mix in the background, gazing at something he sees!