Options for object tracking
How about a plastic plate on each car with large, clearly readable numbers and letters? Say, one on the front, and one on the back. Let's call these 'number plates'.

Then have a camera at the entrance of the garage which contains some kind of computer which can read the numbers, and pass them along to another computer for processing.



Then you can record every car that comes in and out.  Perhaps something like this is available on eBay.