That button is supposed to have a cover on it. The button is useless to us as it's only used for Lexus service techs. I don't know the exact reason, but it's also there on my girlfriend's 99 toyota solara too.
I've added the K&N filter to my RX300 and my gas mileage started to increase. Before I put it in I was getting 18.4 and by the time it stopped going up I had it up to 20.1 and now it's back down to 19.5. Mostly because I let my car warm up for a few minutes every day now as the weather up here in the northeast is getting colder by the day.
Good luck with the RX dude.