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For more information on this, take a look at this article:
Bluetooth: How to set up your Apple Wireless Mouse or Magic Trackpad
When a Bluetooth enabled device is “discoverable,” other Bluetooth devices can detect, pair, or connect to it. An Apple wireless keyboard or mouse will flash its indicator light when it is in discovery mode.
Apple devices will turn off discovery mode after approximately three minutes to save battery life. Mac computers are always discoverable, unless you disable discovery mode in the Bluetooth preference pane.
For more information, take a look at this article:
Wireless input devices: Bluetooth frequently asked questions
Hope this helps,