Hi, I have had an RX300 for over three years now and have had this light come on around half a dozen times. The first time I called the Lexus dealer and was told if the car was running fine to ignore it as it was usually a faulty sensor. Strangely the light was on for a few days and then went out not to appear again for many months. It has since come and gone a few times, funnily enough, it came on for two trips yesterday but has now gone off again.
So, whilst I have absolutely no idea what it is, (the owners manual was not helpful either,) hopefully I can reassure you that it may well disappear and thus the thought of a big bill with it!
Fingers crossed!