I returned from a week's holiday and collected my car at the Airport parking lot. On the way home I struggled through floods and within a mile of journey's end I got a warning of tyre pressure loss. Checked the pressures this morning and 3 tyres read 35 psi and the other 32 psi. All were set at 36 psi several months ago.Re-calibrated the pressure system after pumping up the tyre  and all seems well. Couple of questions. Is 3psi difference generally enough to trigger a warning? I know the system works on wheel revolution patterns via the ABS and therefore could the trip through the floods have affected it?

The difference was 4 psi on the tyre that was down to 32psi.

 

It did the job, tyres no longer at the pressure you set the TPMS at.

They are apparently quite sensitive to a small loss of tyre pressure.

 

If it happens again check the low pressure tyre out for nails,or the like,in the tread.

the tyre pressure warning light has regularly illuminated on my car even when the tyres are all Ok after checking ! 

"All were set at 36 psi several months ago."!!! Perhaps you should check your tyre pressures more regularly. Changes in temperature - winter to spring - will change the 'at rest' pressures.

I check the tyres on my car and my wife's at least every two weeks. Not just the pressures but also look for damage or anything stuck in the treads. Part of my basic maintenance regime.