Aha! Tony thank you for solving the mystery.
If anyone else runs into this problem, if you accidentally or purposefully hit both the spacebar together with the "v" key just above it (which you can easily do when you are furiously doing fine zoomed-in brush work and panning in different directions using the spacebar and mouse), your next depressed click with the mouse will move your layer. I'm going to assign a control key to that so I don't accidentally do it yet another time. This fat finger problem has been my nemesis.
Fortunately, I did have my last "flatten" in history. I went back to the state just before that and was able to nudge the layer back to its proper place. When I ran into this problem last night, I'm glad I just walked away and asked this group! Now back to brushing out the dark lines ...