As I pointed out in The Elgin Marbles, Greece tends towards a double standard when it comes to 'returning' works of art. Britain legally acquired the Elgin Marbles, but the Greeks now want them, so the Greek government is very pro 'restitution' in this case. A Greek museum bought an El Greco which turned out to be Nazi war loot, so even though any other civilized government would return it to the heirs of the rightful owners ... the Greek government won't, and is anti restitution in that case. What's yours is mine, what's mine is mine - that seems to be their attitude.
Another long-running argument is over a set of Byzantine silver plates looted from Bulgaria, but currently 'owned' by the Benaki, a Greek museum. Because of the dispute over their ownership the Benaki will not be loaning them to the upcoming exhibition on Byzantine art at the Royal Academy in London.
Silver Plates Owned by Greece and Claimed by Bulgaria Will Not Participate in Byzantium Art Show - Bakan Travellers.