Like you already said, someone can tap to a keystroke..
as for a mouse, someone can have a mouse and move around running, copying, deleting something from a distance..
That's the simplest thing I can imagine.
My very hi tech employer has decided that cordless mice and keyboards are a security risk so we've had to revert. Has anyone any idea of the signal strength etc of any sort of cordless device? MORE IMPORTANTLY, while one could conceivably pick up the keystrokes from a keyboard, what is there to pick up from a mouse? Anyone out there work for a Defence or National Security organisation? The reason given to me is that the signal sent is not the criterion, rather it is the potential sensitivity of a receiver!!! Am I out of my mind to object?