Q: Bluetooth mouse pair when computer is just starting up

Hi J.M.B,

Thanks for using the Apple Support Communities! I'm sorry to hear that you are having these issues with your wireless mouse. Generally, if the computer does not detect a paired and active mouse on startup, it will automatically go into pairing discovery mode. If you have any issues pairing your new mouse, you may find the information and troubleshooting steps outlined in the following articles helpful:

Set up your Apple wireless devices - Apple Support

If your Apple wireless mouse, keyboard, or trackpad aren't working as expected - Apple Support

Regards