There's the question whether something is lost property or abandoned property. You'd be allowed to keep abandoned property, but keeping lost property without looking for the owner is in many places considered theft.
A car on your land is quite likely abandoned by the last driver (people don't usually lose cars). But the question is whether it is abandoned by the owner; if the car looks like it has some value then it is unlikely to be abandoned by the owner and more likely that it has been stolen.
I'd report the car to the police; then it's up to them to find the owner or not. If they can find him, and the car was not abandoned, but actually lost (unlikely) or stolen (more likely), you have the satisfaction of being an honest person helping either a very stupid car owner or a crime victim to get their property back. If they can't find him, usually the property will then belong to the finder.