Your mouse was designed with a space where you can store the Unifying receiver. When you're not using the mouse, you can store the receiver inside of it.
To locate the storage space for a Unifying receiver in your mouse:
Flip the mouse over and slide the battery cover off.
Locate the small rectangular slot next to the battery compartment.
Slide the receiver into the slot. It will fit facing either way.
Replace the battery cover.
TIP: If you have an extra receiver from any Logitech Unifying device you aren't currently using, this is a great place to store it.