Currently having exactly the same problems with my 05 GT... went off in the night, couldn't turn it off, drove around for a while (in my undies) until eventually it stopped.
Same thing with the long indicator flash followed by two shorter ones.
The beeps stopped about 18 months ago for no apparent reason but only started working again few weeks back. They stopped again until the night the alarm went off whereafter they sounded again as normal. Currently they're not sounding and so the saga continues.
Now the Cruise control isn't working.... Is this related?
Very interested to hear anyones insight as of how to put these Alfa demons to bed!! Ta