For smart key users:

It appears without a techstream (more on that below) you won't be able to stop your buzzer from going off when your key is inside the vehicle.

Your key sends a signal to the Certification ECU (Smart Key ECU) that it is inside the car, which in turn signals the "Combination Meter Assembly"(Gauge Cluster) CPU to sound the buzzer. All communication is done via can bus signal, there is no physical wire or physical switch that can be disabled to circumvent this system, but.................

There are cables that are available online that will allow you to hook up a laptop with certain software to your vehicle to disable it. I'm not going to link you to how to do it, google is your friend, searching toyota techstream could be useful.

I still can't tell you 100% you will be able to disable this, but it's a pretty powerful solution.

-Drew