Did you check if you left some lights on? Like the glove box or some other mods/accesoiries. These lights do not use a lot of power, but if your battery is almost empty, it may still drain it.
Did you install any aftermarket parts, e.g. alarm, ... Probably it was not installed properly.
How often do you drive your car? If you only make short trips, your battery may not get the chance to charge completely.
I would by a battery checker, that can be plugged into your car lighter, such that you can monitor the battery status.
In addition, to give yourself some more peace of mind, you can buy some emergency battery charger(some can be only used once), the current ones are very small, and not too expensive.
Start monitoring your battery for a few weeks, and if the battery is deteriorating go back to the dealer or find another dealer.
Remember you have the top Lexus and you should demand top service.