Hi, I am surprised that just leaving the interior courtesy light on caused the battery to go flat to the extent that the car would not start.
I no that on the GS450h if the battery power drops down to a critical level the car will start to “ load shed “, in other words it will start to turn of non vital electrical equipment to preserve the battery so that the car will still start.
Is the battery on your RX the original?. If so you may want to have it checked out, as it could be getting on to being 6 years old.
Hi Wendle, thanks for your advice - I will check out the battery on my return