I rebooted the Nook Touch, as recommended.
The Nook was at 43%, having gone from 100% to 68% to 43% in 2 successive nights of non-use (yikes!) and no wifi.
After rebooting, I read for awhile in the evening and when I put it away to sleep it was still at 43%, a good sign. Woke it this morning after a whole night of sleep (me and it) - still at 43%!! Yes!
So, it appears that rebooting it does solve this problem. One wonders if the battery really is draining during these episodes, or if the Nook just THINKS it is. Also, I sure hope I don't have to do this after every time I recahrge it. We shall see...