The alarm horn on my OB is under the wiper cowl which is the black plastic running across from side to side visible with the bonnet open. Take the wipers fittings off the splines, then find numerous clips holding the cowl in place. The horn should be visible underneath. Bit fiddly but not too hard. You Foz may/should be similar. When I did mine, the horn mounting bolts were rusted, as were the two bolts holding the horn together. I had to drill mine out. After that, open the case and you should see a rechargeable 9v battery.
Before doing that, I'd suggest checking all door contacts, including bonnet and hatch that they are "telling" the alarm a closed signal. Also, I think the interior movement sensor can be adjusted for sensitivity.