I've had the same problem twice. The first time it took longer than 2 days to drain and it was the removable facia from the radio. As long as that is removed after a journey everything's fine.
The second time sounds more similar to yours as I was losing the charge in two days, then eventually over 24 hours. It turned out to be the alarm siren. There's a little battery inside the siren which is supposed to sound the alarm if someone disconnects the car battery while the alarm is enabled. This little battery was obviously knackered (9 years old at the time). My alarm is factory fitted so if yours is too it could be the same problem.
The easiest way to check is by simply locking the car with the key rather than switching the alarm on. If the battery keeps it's charge then at least you know the issue is alarm related. You can then remove the siren which is under the plastic cover on the drivers side of the engine bay. Then switch the alarm on and check the charge in a day or two. If it's not that it could be a problem with the alarm brain which is near the back of the radio.
I always intended to replace the little battery and put the siren back on but the battery has tabs on and I haven't been able to find one (I haven't looked that hard if I'm honest). The alarm still activates, red LED flashes etc. Lights flash if the alarm goes off but there is no sound.
You can buy new sirens here but they're expensive at £89, Abacus Alarms:
Alfa Romeo GTV alarm
They have a page dedicated to GTV's which is pretty good.
One last thing to try is the light in the boot which can be switched on when you're getting stuff out of there.
Also the battery could be knackered!
edit: My car is the same as yours, same year and everything so I'd put money on it that the alarm's at fault.