Does a Magic Mouse, trackpad

Do you turn the switch off on your mouse and keyboard when you turn off your Mac?

If you don't turn them off then, yes, they are using power even if your computer is off.

They may be using even more power because they are searching for a device to connect to.

But if you are turning them off on them when you turn off your Mac they should not be using battery.

They do use a relatively small amount of power if not turned off; better to have rechargeable batteries so you can always keep a pair ready for action. I have 12 rechargeable batteries and rotate their usage. All the same they still run down a little.

For regular batteries anything from supermarket bargains to dime a dozen stores selling dated stock or fake copies will generally give you poor service. And if the copies explode, cleaning up the mess is awful. If you prefer to use regular batteries use Energiser, Eneloop, Duracell or Apple.