Annoying low tire pressure indicator is always on

I have a 2008 Lexus RX350 AWD. It is one year old now. It has been behaving well until recently the low tire pressusre indicator on the dash board keeps on when ever I start the car. I checked the tire pressure for all the tires, and they are 32psi. I could not find any flat tire, no holes either. I was just wondering why the vehicle is complaining. It is really annoying.

Any advise on this? Thank you very much for your input.