That --- reading indicates that there is no signal for that tire pressure sensor 'slot'. It means the sensor has failed, most likely due to a dead battery although it is remotely possible that the sensor itself has failed.
The 'low battery' warning is actually transmitted from the sensor, which means the sensor is actually working and sending information via RF. When the sensor sends nothing, or is not present, the LCD readout uses the filler '---' because it has no information. At first this seems backwards because we tend to think the display or ECU on the bike is making all of the choices but the low battery warning is actually generated in and sent from the sender itself.
As others have mentioned, potted sensors can have the battery changed; we had one open at tech. day last spring although we did not actually remove the battery. The battery itself is not potted, meaning the epoxy makes a shell and seal around the chamber the battery is inside of. But there is a learning curve to changing the newer types and as I have not gotten to the end of it, I cannot really help you out very much. Of course you can R&R the sensor while the tire is on the wheel, and while the wheel is on the bike at least.
Brian
Well, solve one problem and another appears. I have noticed over the last few trips that my front tps (tire pressure sensor) was reading "---". I have seen this happen in the past, but it is very rare. No longer. Yesterday it was absent on my way to a destination, but returned after the next start on the way home. Today it was absent my entire trip, across 4 starts (initial start, break time, stupid draw bridge, then for gas). Never a low battery indicator. Rear one works fine every time.
These are the newer model (potted) sensors, installed under warranty. My first ones failed with low batteries on 4/2014 with 9K miles (were only 3 years old), with months of random battery warnings, depending on temperature)- very disappointing. So these are now 4 years old with 32K miles on them, so it was looking pretty good (maybe they really have fixed the design).
I don't recall many, if any, posts showing these symptoms. Usually they are people who have gotten low battery warnings, or bought their bike used, with already dead sensors. I am thinking maybe the centrifugal switch is sticking?
Starting to document each occurrence with photos as proof, if this escalates.