If you hear any squeaking sound from any of the brakes of Yamaha R15, don't panic yet. Normally the sound is heard when you are applying brakes on a steep descend. At times the sound is also heard while you move the bike in the morning. There is nothing to worry about this. It is pretty normal. The squeaking sound in most cases will be made by dust entering the brake system. This is found more so in the case of back brakes of R15.
You don't need to visit the service centre to fix this. Give a thorough wash to the brake area with water from a hose at a reasonable force. Apply water all over the brake area. It would be better to apply some car shampoo in the area. The sudden forceful gush of water removes the dust that is making the sound. Allow the area to dry completely. Let it stay dry there for at least 2 hours. Now do a test run and see whether the issue is still there. If the issue persists, you may have to visit the service centre and ask them to dismantle the brake and check for any malfunctioning.