False alarm! Never fails, as soon as I post with a problem, I Google it and find the answer. It seems there is a "floating" shutter button. I don't know how I enabled it but to DISable it, long press till a - (minus) sign appears and tap it.
One situation would be when you're using the phone one-handed and trying to take a selfie picture. It may be a more stable picture if the button can be moved up a bit etc.