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... is right, I was referring to a spatial region, and by leaving I meant losing contact/interaction with other parts.

Wouldn't there need to be a force to make the object not keep going out of the system? The force would then cause more movement.

Yes, I know, also if a piece of matter moves in a straight line, it will leave the system. Is it correct to say that a system with moving parts will always lose matter/energy in that way (or another)?

Always leave a system with moving parts. If the objects try to go in a straight line then there would have to be a force making them stay in the system, but forces require more motion, which would then require more force to not go in a straight line. (note: I'm only 14, so I don't know that much, but I'm pretty sure electromagnetism, gravity, etc. haven't been studied enough to assume them to be axioms).