The levels of delta-v we're talking about here aren't nearly enough to knock these objects out of the solar system. When they leave the Moon or Mars they're st

The levels of delta-v we're talking about here aren't nearly enough to knock these objects out of the solar system. When they leave the Moon or Mars they're still in orbit and will remain in orbit until they collide with something. In the case of a moon rock that means either hitting the Earth or returning to the Moon in what is likely a single orbit so 2 weeks. In the case of Mars it will either hit Mars again or the Earth. Or possibly the Moon, one Mars's moons, or an asteroid but probably the first two. The odds of this collision taking place on any given orbit around the Sun are low but the object isn't going anywhere and eventually, possibly millions of orbits later, it will end up hitting something. So no, the probability that it will eventually hit the Earth doesn't really go down with the distance in this case since we're only talking millions of years for a collision to occur rather than billions.