Car sensors seem like they would do nicely as an addition, but I don't know what really exists in this space. I'm down to maybe build something myself if there's nothing reasonable out there already, but I have very little experience with Arduino (lots with raspberry-pi, but that's not so great on cost/power).
Ideas:
Both my cars have some sort of bluetooth, but no wifi. If I could detect the presence of the bluetooth device, I would know that car was present. But of course they power down and disappear when the car is shut off. Would probably have to poll frequently to "catch" the car arriving or leaving before it's powered down or out of range... and then which is it, arriving or leaving? Maybe useful, maybe tricky.
A garage "distance sensor" / "range finder" / "metal detector" might work. I could put it on the ceiling and detect when something of the same height as the cars was present. Works all the time, but maybe subject to false positives (boxes in the way of the sensor, people walking under it, etc?) and doesn't cover parking in the driveway or street.
A small zigbee/zwave device put in each car? Just something that says if the device is present/detectable, not one that needs to do anything. Ideally doesn't shut off when the car is shut off, so it's always easy to know the device's actual presence and not just "present and powered on". WiFi would work too, it could just join the network and I could use Joel's presence sensor to see it... but that's maybe getting power-prohibitive for an always-on device, it would probably need a battery and be able to charge from the car when running.
Does anything like any of these ideas exist already?
I have used the iris key fobs to do this in the past. Samsung also makes one and I have seen someone replace the batteries with a AA holder so it lasts longer
Head over to the Everything XBee thread. They have a Zigbee driver built and adding a python script gets you presence, and the XBee has a very strong signal.
XBee gets connected to car battery, and reportedly works great
It is not a presence sensor. But, It will act like one. You can put one of them in a car. When your car approach your house, it will be detected like a Bluetooth beacon.
In a garage, would a contact sensor work? Maybe placing it on the ground or against a wall? Not sure any of the magnets would be powerful enough, but maybe a place to start. Either that or some sort of a pressure plate. Like when you drive on it, the plate makes contact and triggers a scene or something.
I have found an Xbee3 plugged into my car's usb port to be the most secure as it is 100% local and only powered when the car is running. It also has much better range than the SmartThings arrival sensors.
You can also take the battery from the SmartThings arrival sensor and cut an end off a usb cable and solder the usb wires to the battery contacts of the SmartThings arrival sensor and use the same as an Xbee3 but just not as much range.
Added safety feature is it will open the garage doors whenever the car starts to prevent carbon monoxide poisoning. If the kids get a hold of the car's remote starter or get into the garage and the keys are within range and get the car started or if a bedroom is above the garage (Should be sealed but...)
It only takes one time.
No for controlling the garage door I only use the Xbee3 arrival/depart (open/close)
It also turns on lights and unlock the garage house door.
For showing the car present or away I use a second sensor my old SmartThing arrival sensor with the Cobra's battery mod mentioned above.
It's nice to see if the car is home but not really needed. I just had extra sensors laying around.
Using a sensor that is always powered on is not the best for securing the garage door and it would not work for protecting from carbon monoxide from a car being started in a closed garage.
Xbee3 out of range or off closes my garage door combined with motion sensors in my garage going inactive rule.
I always try to close my door myself before I leave the Xbee3 serves as a backup for closing.
I have also been using @Cobra solution for over a year now. First with Hubitat and now with Home Assistant. Works perfectly and haven't had to replace the batteries yet. Battery is at 84%..assuming it's accurate.
. Check out the old thread in the ST forum that 
