Hmm, I was stalking the (avian) birds today, and encountered the problem again. This time the centre focus point was lit up and focusing as if I just had spot focusing on. Bit annoying as its the first time Ive got tits in the frame.
However a bit of fiddling and it seems that a certain method of selecting the focus points might have fixed things.
Basically on the 7D there are at least 2 different ways to choose the focus points (probably more). Theres a quick menu button you can press which brings options up on the LCD, and you can choose through that way, which is what I was doing. You can also press a button and then the m-Fn button whilst looking through the viewfinder which cycles through the options. The quick button method did nothing when I was in the field, but just by pressing the focus point button and then m-fn to cycle through the options once seems to have cleared the lock.
So maybe I am pressing random combos (99% sure I didnt) whilst moving the camera, or perhaps the camera getting cold too quickly is doing something inside. Ill keep my eye on it, hopefully will end up out in the forest in the next week sometime