Stolen sculptural fragments returned to Italy

Relief removed from the gardens of the Villa Borghese. Source: MiBAC

Among the objects returned by Christie's to Italy last month were two fragmentary sculptures [press release]. One was a relief showing a satyr and maenad that had been removed from the gardens of the Villa Borghese in Rome in 1985. The second was a sarcophagus fragment that had been removed from the catacomb of San Callisto in Rome by 1982.

Sarcophagus fragment from the catacombs of San Callisto. Source: MiBAC.

The press release from Christie's stated:

The works, that had been acquired in the past in good faith, were more recently identified as not having the required, verifiable title, export or provenance details needed to proceed with a sale and as such were returned voluntarily.

How did these two pieces end up being consigned to Christie's? Where have they been residing over the intervening three decades? Who has been handling them? Who has been passing the sculptures on 'in good faith'?