If a private citizen or a company disposes of a piece of property with no expectation of its return, then it can be collected by another person without hindrance. It is considered "abandoned" per UNITED STATES v. REDMON.
Thus, the intent to relinquish ownership and abandon trash is
tantamount to “throwing away” a subjective expectation of privacy in
it that society accepts as objectively reasonable. In short, when it
comes to abandoned property, “I know it when I see it,” Jacobellis v.
Ohio, 378 U.S. 184, 197, 84 S.Ct. 1676, 1683, 12 L.Ed.2d 793 (1964)
(Stewart, J., concurring) (identifying what constitutes “obscene”
material), and when the police see abandoned garbage which has been
left unsecured in its usual place and at its usual time for
collection, it is theirs for the taking..
Since Musk has stated that there is no reasonable possibility of its return and since he is no longer in communication with it, it can be considered (for all intents and purposes) to be abandoned garbage unlike, for example a privately owned orbiter or a communication satellite.
"We estimate it'll be in that orbit for several hundred million years,
maybe in excess of a billion years," Musk said before launch. Tuesday
night, he tweeted: "Third burn successful. Exceeded Mars orbit and
kept going to the Asteroid Belt."
SpaceX's Tesla's out-of-this-world view