I had a similar problem. I was driving over a one of the bridges that connect from where I live to Portland and the I get the warning light. I get off on the next exit, visually check the tires and check the pressure in them, no difference. Hop back in the car, reset the system and start driving again.

About a week later, I am driving over the exact same bridge and it goes off. I pull over again, check everything out and don't notice a single difference as last time.

So, I figure it has something to do with the bridge and maybe there is a dip or something in the road that is setting off the sensor. A week later, I walk out to the car and it has a flat! Fill the tire with some air and drive down to the local tire shop to get it patched.

I had a long thin nail stuck in the tire for sometime and it was slowly, very slowly loosing air. Whenever I would drive over the bridge, there were metal dividers on each section. I think that when I drove over it, it would jar the nail a little more until finally the leak became bigger. So, even though you don't think there is a leak in your tire, you may certainly have one.

__________________
Jeff
Current:
'07 Jeep Wrangler
'06 BMW X5 3.0i

Former:
'12 WRX; '08 WRX; '06 BMW X5 4.8is Dinan S1; '02 BMW 530i Dinan S2