In all likelihood, it's the battery in the wheel sending unit that is causing the TPMS light to illuminate. But no way to check unless the wheel is off and the tire(s) removed. Then the new battery has to be sync'd with the receiver unit and there have been some calibration problems with new sensors and the receiver. However, if the TPMS light is on for that reason, sooner or later your low pressure sensor will not work as well.
Could be, but the bike just turned 3 years old and it's only got 6700 miles out. The batteries shouldn't be bad already, should they? However to test the theory, would removing the air from tires and then starting the bike to see if the low pressure sensor turns on, be a good test?