I could be wrong on this, but I think the TMPS triggers a warning when the pressure drops below a % threshold of what it thinks is the normal pressure. The 'normal' pressure varies from car to car depending on wheel and tyre options. I think it 'learns' what the normal pressure is because it starts off with zero pressure before the tyre is fitted, then the tyre is fitted and inflated, then the car is driven around for a bit and the ECU gets stable readings from all four sensors, it decides that's the correct pressure assuming that whoever pumped the tyres up did it properly.
If, like me, you get some numbnuts that over-inflates the tyres ('cause they think you keep pumping air in until the warning light goes out) that pressure gets stored as being normal, and when your tyre pressures are correct, it's seen as low pressure compared to that abnormal benchmark. Try letting air out of all the tyres, pump them back up to the correct pressure, drive around for the ECU to learn the new 'normal' pressure, then it should be OK. My normal pressure is 36 PSI, the low pressure triggers at about 31 PSI. I check my tyre pressures regularly, usually they haven't fallen by more than 2 PSI between checks.
