Don't mess around with additional alarms tbh - Just something else to go wrong and unless it's installed completely perfectly you will have problems.
The stock alarm and immob is perfectly good enough.
Aside from hiding your keys you're just worrying yourself, it's very unlikely to happen and if someone is desperate to nick your car they will do so, with or without the keys.
I had a tracker on a motorcycle (Street Triple R fully specced kept outside in a communal parking area secured with ground anchor) and aside from drain the battery and the tracker company call me to say "your bike has been stolen" when it's actually ran flat on battery, it never was any use and to be honest I think it's a bit of a gimmick!
Besides if your vehicle was stolen, abused, used for a crime perhaps, or partially stripped/damaged, would you really want it back?
If you were serious about a tracking device you'd use a GPS and RF tracker that you buy separately and hook them up to web-traceable interfaces.. costing a lot less than the £400 or so for a regular tracker system fitted plus the £200 a year "subscription" to make the system active.
Outside of keeping your keys in a safe place you just get into a big circle of worry and paranoia.
I have my keys hidden and video surveillance on my driveway (we have 3 cars at our shared house) so benefits everyone, living on a main road. Outside of that, you have insurance.
