I would be interested in seeing the masks, the head stamps on the bullets and the weapon parts to see where they came from....
If they were found near the eastern coastal areas, they may possibly have come from POW's from Gallipoli. There was at least one German Pioneer company and a number of German machine gun crews there, and while I can't find an exact number it seems around 500 Germans were taken prisoner in the area, a mix of soldiers and sailors. It seems they were shipped to a POW camp in India by the British, but it's likely they were held for at least a short time in Greece before being moved and you may have found a holding area.