Everything is a contract including basic human rights. By birth, you have not signed any. You dont have anything that you can lose. But usually you would implicitly sign "right to freedom" and "property laws" with people around you.

So you did not lose right to move in private properties (because you did not have any) but you gain right/assurance to move freely in public properties without danger to your person.

If aliens invade earth and set their own property laws it would not be stealing. It would be nothing. Because we dont have any contracts with aliens that we can claim aliens have broken.