You could always drop by the dealership on Monday...I won't be able to sleep at night till I know what this is...
According to another poster on the Club Lexus forum, the tank could be the intake resonator.
It's intended to reduce intake air noise.
The white tank under the battery is the 2nd windshield washer tank, connected to the main one on the passenger side where you can refill. The hose connecting the two tanks is run along the front fender but kind of hidden behind the black plastic cover, you just have put your fingers behind the black plastic cover and poke it out to have a look then easilly snap it back. Mine is broken, causing a leak. When I refill the tank, on the passenger side, I saw liquid spilling out on the other side underneath the car, so I was curious and found out.
2009 Rx 350: What's The White Plastic Tank Under The Battery ?
in 04 - 09 Lexus RX330 / RX350 / RX400h
Posted
The white tank under the battery is the 2nd windshield washer tank, connected to the main one on the passenger side where you can refill. The hose connecting the two tanks is run along the front fender but kind of hidden behind the black plastic cover, you just have put your fingers behind the black plastic cover and poke it out to have a look then easilly snap it back. Mine is broken, causing a leak. When I refill the tank, on the passenger side, I saw liquid spilling out on the other side underneath the car, so I was curious and found out.