My sister had her car broken into last month and discovered her car's stock alarm from the dealer didn't have anything to detect broken glass. The door was never opened and thus the alarm never triggered. I know there's a variety of alarm sensors you can extend a system to cover (level/tilt, broken glass, motion, presence of a body in the cab, etc). How would one go about discovering what sensors they can install and the capabilities of an existing alarm?
If you said what make, model and year the car was it might help, if that model was available with motion, ultrasonic or tilt sensors it will be possible to retrofit them.
Retrofitting a factory option can be as straight forward as installing the necessary parts and plugging in some connectors. Sometimes you will need to get certain control modules reprogrammed by a specialist.
If it was not a factory option then you will probably have to install a aftermarket system. As you've already got an alarm system it seems a bit horrible to have extra key fobs or sirens etc. You can buy systems that work with the factory keyfobs/central locking and immobiliser system so there is no extra hassle involved.
You can get extremely cheap and low-power motion sensors (PIR type). They consume current in the microamp-milliamp range (5V) when active, so it won't run down your battery.
When triggered by motion, they shoot a 3V3 signal out, which can be used to trip an alarm. They're small; 35x35mm. Just pull a wire from 12V to a voltage regulator (7805 is very cheap and reliable). If you can't find an alarm system that is tripped by 3V3, it's easy to amplify to 5-12V with a transistor.
The 3V3 signal lasts about a second, and the PIR will auto-reset. I've used HC-SR05, but I recommend something less sensitive, it can pick up movement from 5-7 metres. Ask for more detail on Electrical Engineering / Arduino stackexchange if you need a controller for your builtin siren.
There's also the option of using sensors which detect loud noises and kinetic shock to the car frame (like seismic sensors, but not seismic sensors). I would mix a lot of different types of sensors together if I needed a reliable system. Much of it can be deployed under the dashboard or beneath the trunk if you don't have the knowhow to embed it real deep.
I know a mechanic who often deploys motion sensors in the vans of e.g. carpenters, because thieves just love cutting out the window panes and just loot their vans this way. It's very reliable, and I am (pretty) sure a PIR doesn't see through the type of glass used in car windows. I could test this if asked to, but meh...
If you want to get really freaky, get 3-4 of those lady-alarms with a string that can be pulled in the likely event of a rape attempt. They're 120dB at 3V3, so a few AAA batteries will do. Trip them instead of the car siren, a thief will have his ears destroyed in the enclosed space, and people who hear them will respond to the rape attempt of your sister's car.
Strobe lights at specific frequencies (about 50-60 blinks per second) mildly (or seriously if the thief is epileptic) disrupt cognitive functions. Really unpleasant stuff, I did it to myself a few years ago. Yellow is the most effective colour.
And then there are of course gas systems which use bear repellant gas, or real tear gas if you can get it legally. Gas is NOT funny (I am a jew), but it is VERY effective. JUST LOOK AT THIS GUY -> https://youtu.be/vBIIcGRFHZs - this is the coolest solution to your problem. I guess your sister won't think it's cool, but it's much less cool to lose all your stuff. Tell her that if she complains.
The likelihood of these countermeasures affecting your sister is almost 0.
There’s a pretty useful video this fella posted on YouTube that details the process for extending a factory alarm to gain additional threat coverage.
He locates a 12V constant line, the ignition line, the and the door switch line and makes all the necessary connections to get the sensors to simulate an open door.
From the video’s description
You must!! purchase Shock sensor as the main integration device!!
How to install Motion Sensor, Glass Break Sensor, Impact Sensor, or Tilt Sensor onto your vehicles OEM alarm without having to buy aftermarket alarm.