It’s not saying you have a flat tire; it’s saying you have a tire with pressure below the minimum which I have read is 27 psi although I don’t think the manual says that. The signals can also be interrupted by spurious radio signals. There is a place between Austin and Dallas that my warning often shows up but it will go off in a few miles. I don’t even bother to stop and check anymore. Of course the TPMS system could be defective.