According to USA today (reliable?), the police were doing a crack-down on illegal activities such as drugs and in the course of this raided the camp. From the website
"..the blonde, pale-skinned and blue-eyed girl stood out. She bore no resemblance to the Greek couple, a 39-year-old man and a 40-year-old woman, who were arrested and charged with abducting a minor. Police said DNA testing confirmed that they weren't related...
Police are trying to establish why the girl was living with the couple, who are also accused of falsifying identity and family certificates — with the mother claiming to have given birth to six children within a total of less than 10 months...
The suspects allegedly offered conflicting accounts — that the girl was found in a blanket, was handed to them by strangers or had a foreign father.
The police statement said the couple claimed to have a total 14 children, and had registered different numbers with authorities in three different parts of Greece. Officers found three minors living with them in the settlement who appear to be their children — although that hasn't yet been verified by DNA testing.
Charity director Costas Giannopoulos said ...
"There is much more to investigate, there are other registered children that were not found in the settlement, and I believe police will unravel a thread that doesn't just have to do with the girl."
If this is true, they have already been charged.