Don't know about the Brera but on the 159 you press the button on the roof console (which I think then starts flashing) and then leave the vehicle and lock the doors. This disables the alarm- useful if you park in a safe place with the windows partly open in hot weather, where otherwise the volumetric sensors would trigger the alarm
EDIT: On re-reading the manual, this process may only disable the volumetric sensors - but give it a try
To make sure that the protection sensors
are working properly, check that windows
This function can be cut out (for example
if you leave animals on the car) by
pressing button A-fig. 12 on the front
ceiling light within 1 minute after instrument
panel turning off.
When this function is off the button led
will turn on. Volumetric protection/antiraising
sensors cut out shall be repeated
at each instrument panel turning off."
Copyright Alfa Romeo Brera Owners Manual