I wanted to let everybody know, since it isn't commonly known, that pretty much all cars past 2005 contain a hard drive which records location, acceleration, and other data. They are commonly used in car theft cases and insurance claims. From the forensics guys I've talked to, they noted that some modern systems phone home to the manufacturer, but that data is rarely made available to the police. You pretty much cannot buy a modern car without one of these hard drives. Stay safe and if you have questions I can try and answer.
I haven't heard of anyone using this to prove that someone was at a protest ect, but I would expect it in the future. The cops do need a warrant for this and because getting to the drive is a fairly destructive process, they will (generally) actually get one.
My knowledge on them is limited, but as I understand it only newer cars transmit the data, and only ones that have the cellular package. If you car does not have any kind of cellular then it most likely doesn't transmit outside of the vehicle. Most car manufacturers as I understand it don't transmit the data home, it's usually just the luxury brands, but it is very hard to tell who is and isn't. Most cars that have on-star, or a similar system, will transmit data in the event of a crash, but I do not know, and I don't know if anyone knows, if it transmits telemetry beyond crash location. The hard drives are typically 2.5 inch high vibration protection drives, so they will hold from 500gb to 2tb. They will just tape over themselves once they fill up, but I expect that some may filter what data is over written and what isn't. Realistically, even a 500gb one can hold a couple decades of logs, as they are just text files with time stamps. As with everything with these, what's recorded depends on the manufacturer. Typically you can expect the following information to be logged: car start, car speed, car location, car settings, cruise control, acceleration, engine error codes, door status, window status. The data does take someone trained to access it, but not overly so. Most cities will have a joint digital forensics lab that will have the capabilities, or can send the hard drive to someone with the capabilities. The actual data on the drives may be encrypted, but it also may not. Again, it depends on the manufacturer. Insurance companies are the other big user of this data.