Could it be possible that a tyre sensor could go wrong or need replacing if it has not been used for a while? I had the tyres changed on my 2003 BMW 330 CSI and they only had three metal valves, so one sensor was not fitted as it had to have a rubber valve. I finally got to go to a good tyre place where, on fitting the sensor with a metal valve and putting it in, the alarm will not stay off.