Thought I would add my tuppence work to this as the Lexus dealer were hopeless. I had the same problem, but intermittent a few months back. It happened after I had parked on a really steep hill, so I thought the fluid levels were out, or maybe the sensor got scoobied. Anyway, light went out after a few weeks and was OK for a few months, then back on again about 2 weeks ago. I called the dealer and asked if they knew why the dash light was coming on when my levels were OK, the sensor seemed to be working (i.e. I could unplug it, short it and the light still stayed on), and asked if there was any other reason the dash light would come on. I happened to mention I had just replaced my rear pads and their service desk said "maybe they have missed the brake wear sensor wire" ... doh, there isn't one on that year of IS.
Anyway, their solution "bring it in and we'll look at it".... yeah right.
So, back home, pulled in to the drive and asked my son to stick his foot on the brake pedal. Yup, high level brake light was not working.
A quick jiggle about, the bulb was just loose, not blown. Hey preston, dash board light out.
To access the high level brake light, check the owners manual, it shows you a piccy how to do it. Basically, 2x flat head screw drivers from the inside of the car, prise it up gently. The relector part then can be prised our of the housing for easier access to the bulb holder. Oh, and the bulb is a E4 2EL 12V 16W push fit on a 2000 model IS. Not as the Lexus dealer told me (and they looked up my chassis number) a 12V 18W or 12V 21W.
Honestly they are muppets.