Cars are often parked in driveways and these driveways are, obviously, part of the debtor's owned or leased property. However, repossession companies routinely take cars from driveways. They often attempt to obtain and usually succeed in obtaining a sort of coerced consent from the debtor in this way: the repo guy will often say that if you don't give him the keys, your car will be damaged, or lie and say that you have to give him the keys. He might also start a disturbance to embarrass the debtor into complying. Since repossessions often happen late at night or the in the early morning, and the people doing the repossessing are often experienced in the subtle and arts of intimidation, this is often effective. The bottom line is that a lender can't take a car from a driveway if the debtor does not allow it. If they don't get consent, they must take the car from public property or get a court order allowing them to take it from the driveway.
Now, there's no point in unnecessarily delaying the inevitable: if you can't pay your car loan, the lender has a right to the car. However, there are rules for a reason--to maintain peace and good order--and if these rules are broken there are consequences, including at times significant statutory damages for the debtor. So, you might want to consider keeping the keys if a repo man visits your driveway in the middle of the night.
*Note: If your car has been repossessed in Massachusetts, we might be able to help. However, due to high call volume after I posted information here about Massachusetts car repossession, we must first receive the completed form found here: http://www.mass-legal.com/repo_quest.asp. We will review your matter confidentially and free of charge.