Interesting stuff from Beemer World! However, this still does not help identify the low pressure level at which the Rocket 3 TPMS warning light will illuminate. I found some USA NHTSA info online that suggests a TPMS must warn [the driver] when the pressure in any single tire has fallen to 30 percent or more below the vehicle manufacturer's recommended cold inflation pressure. It appears that this threshold was subsequently revised to 25 percent. For the Rocket 3 recommended front/rear tire pressure (42 PSI or 2.9 bar) I would expect the TPMS warning light to illuminate when tire pressures (front or rear) drops to somewhere between 29.4-31.5 PSI (or 2.03-2.18 bar). EU may have different (but likely similar) thresholds.
Low tire pressure is one of the most hazardous situations that can happen anytime during a ride, not just at the start. You have only two fist sized patches connecting you and your bike to the road that rely on proper inflation to keep you on your path, and to stop when you need.