Your K400r has a space for storing the Unifying receiver when not in use.

To locate the storage space for the Unifying receiver in your keyboard:

1. Flip your keyboard over and slide the battery cover off.

2. Locate the small rectangular slot on the underside of the battery cover.

   

3. Slide the receiver into the slot. It will fit facing either way.

4. Replace the battery cover.

TIP: If you have an extra receiver from any Logitech Unifying device you aren't currently using, this is a good place to store it.