I'm sorry but I have to insist. The MASTER CAUTION light is not linked with RPM, and it is very easy to check it out. RPM is just linked to its own light (RPM) and audio. Have the copter ready to flight with all caution lights off, and cut down the rpm. Yoy will have the audio advise and the RPM light will turn on, but NOT the MASTER CAUTION light.
The same happens with the FIRE warning light, that is independent of the MASTER CAUTION light.
So MASTER CAUTION light should turn on whenever any light on the CAUTION PANEL comes on, to drive your attention to that panel. Once the light has turned on, you will need to reset it manually to switch it off.