Plastics are energy-rich substances, which is why many of them burn so readily. Any organism that could unlock and use that energy would do well in the Anthropocene. Terrestrial bacteria and fungi which can manage this trick are already familiar to experts in the field.
The story struck the depressingly familiar note with which true stories ring in the tried ears of experienced policemen. No one queried it. It was in the classic pattern of human weakness, mean and embarrassing and sad.