My Beater Boxster has been double beeping since the day I got it. I recently purchased a Durametric cable to troubleshoot a CEL issue. During the diagnostics the test results produced information regarding my alarm system. I haven't even begun looking into this yet as I'm still messing with my iPad/iPhone interface.
Check out my post on the Durametric and you'll get an idea what alarm parameters are being tested.
Good luck!
Beater Boxster ODB2 readout.. Ugh!!