I don't know about anyone else, but my scoreboard frequently moves all over the place by itself. Sometimes, in the middle of the game, after I've already checked the scoreboard, I'll hold down tab and the scoreboard doesn't seem to appear.
In fact, what happens is that it *does* appear, but only a tiny corner in the corner of my screen.
I've found that you don't need to click *on* the scoreboard to click-hold-drag it to a new position. So in the cases that I can't find it, I can still click drag in the middle of my screen until I see it slide in, and then position it.
This bug was introduced at the same time the new scoreboard came out.