There will be no error message if there is nothing being sent from the sensors. The 'Low Tire Pressure Battery' warning only occurs because the sensors themselves are broadcasting that signal; once the battery is completely dead (or electrically removed from the sensor, or the sensor is no longer in the bike) any warnings about that sensor will end. You are right in that going to that screen will show (---) but only until the main battery is disconnected for any reason, then when the system reboots, the entire tire pressure sensor screen will not appear.
Brian
I think the main purpose of the sensor is to tell you when you have a serious problem and need to fix a tire, not to give you a minute by minute display of what your air pressure is. I never even look at mine.
How can you give up on them? Don't you get an error message if the transponder isn't broadcasting? Or do you just get lines when you switch to the tire pressure screen?