bump
any idea ?

It's pointless to me as well, but there are those who like it because they
believe it helps prevent them from accidentally leaving the key in when
they leave (or lock) the car.

The beeper in the RSX is controlled by the Multiplex Control Unit. There is
a switch that specifically tells the Multiplex Unit that a key has been
inserted in the ignition (closed=inserted). I suppose if you removed the
steering column covers, you will find the equivalent switch in the TSX.
Unplug it and the MCU should think there's no key. However...what that
would do to your immobilizer I cannot say.

As an alternative, you could simply unsolder the piezo-electric beeper
itself from the MCU's circuit board. It's the same kind of beeper used on
computer motherboards. Removal would silence all audible warnings.

The MCU is also the under-dash fuse block.