NSR,
Actually, you may or may not get an UNLOCK indication if the prop brake malfunctions. It depends on the type of malfunction.
As I posted earlier, a most common malfunction is slippage. Slippage occurs when the engine is running in hotel mode and the propeller rotates, meaning the prop brake can no longer completely hold the prop in place. If the brake pads themselves are excessively worn, but the prop brake is still in the locked position, the UNLOCK light will not illuminate. The UNLOCK light only 'looks' at prop brake switch position and prop brake position; it does not monitor brake wear.
However, if the prop brake unlocks (meaning the brake assembly itself)without being commanded, you will get an UNLOCK light and the associated CCAS indications.
I hope this clarifies what I posted earlier.
Regards,
BrasiliaCaptain (at least I was a few years ago)