I had almost the identical problem the day after I bought my 2000 RX300. Like you, I had driven her hard and found no problems. Not 12 hours after signing the contract, the daggone check engine light came on! I did a little research and learned that for some reason, this light also comes on when you're out of fuel . Now, here's my theory (feel free to dispell it) --> I had been <1/4 tank and made a short stop. I suspect that the low fuel sensor in the tank was momentarily uncovered thereby setting off the check engine light. I refueled and after about 3 or 4 trips, the light magically disappeared. Imagine my relief! :)
No new problems to report. This vehicle drives like a dream!