The provenance principal as it relates to artwork, and cultural artifacts is a pretty well established process for determining ownership. This should really be what governs if artefacts are considered stolen / looted or were gifted / sold in good faith. I bet there is a substantial amount of artefacts in museums around the world that fail a good provenance search, in this cases they should be sent 'home' if a stronger claim can be made. In this case though it's not clear, 'collected' as stated by the article could mean many things.