Most commercial honey is a blend of honey from many different sources, including different countries. For a time honey from China, which is a very popular part of the blend because it is cheap and very sweet, was banned from the EU because of excessive use of chloramphenicol, a broad spectrum anti-biotic. When the ban came into force, blenders were inundated with offers of large quantities of cheap honey from countries with close ties to China that traditionally had not been honey exporters. I think there have also been examples of similar contamination from Argentina. While large scale blending allows suppliers to achieve consistent taste and quality it does open up the possibility of contamination.
Some plants also produce low levels of toxins in their nectar, which are not toxic to some species of bees and which can result in that toxin being incorporated into the honey. The classic example is Rhododendron but there are others. Experienced bee keepers know to keep their bees away from such plants during their flowering season but I suppose it could affect the hives of an inexperienced keeper. I don't know of any cases of anybody suffering ill effects.
The priest in Alikianos is a very experienced bee keeper with hundreds of hives, all of which he made himself. He has an extensive woodworking workshop and industrial level centrifuges and stainless steel tanks for extracting and storing honey. I'm not sure if he sells it or if he does by what route. Some of your Greek friends in Agia might know him, Ann, and be prepared to contact him for you. At least your friends would be taking home locally produced honey. When you come into Alikianos over the narrow bridge turn right at the supermarket. His house is the first house on the right after the little bridge next to the supermarket.