Wireless mice have fairly good power-saving routines; if they’re not moving, they shut down. Alas, if you’re packing a mouse to bring along with your PC, it may stay awake for the whole trip… at least until its battery goes flat.
You can remove the cells, but then you’re stuck with a bunch of moving parts: mouse, receiver, a couple of AA cells. Now you need a ziplock baggie. Fooey.
Better to take a strip of thin plastic, like a small plastic Post-It flag, and isolate one cell. Make it long enough to stick out through the slot in the case and you’ll have a reminder that the mouse won’t start up automagically.
Some mice actually have a mechanical switch, but if it’s a pushbutton then you may as well insert the plastic strip.
This works for cell phones, too, at least if you’re the sort who can afford to turn the phone off because you’re not expecting any calls.