Erp S5 is a sleep mode. Not a shutdown. So expect a certain amount of activity as the pc is technically still On, just not awake. You don't want to turn off the mouse. If you do, you'll not be able to use it to wake the pc, unless you plan on keeping the motherboard awake. You'll need something awake at all times, or you'll be stuck in sleep.
Sleep modes come in stages, not instantly. S0 is awake. S1, things are starting to get flagged for turn off, first stage of actual sleep. After a few minutes, S2, S3 and the pc has most things now shut down, low power mode. S4-6 sees anything non essential is finally shut down, low powered. Cpu is at maximum low power state. This takes over an hour to accomplish. Bump the mouse, pc wakes up.
You can turn that off by disabling the 'wake on' features which by default leaves the mouse and keyboard active but disables wake on lan, network, etc etc.