Greek tortoises are very good at "hiding" their digging spot. They're looking for a cool spot to be able to cool down their body. Is there any shade in their enclosure? You can give them a cooling soak in water occasionally too. Maybe if they had a nice artificial cave to retreat into they wouldn't bury themselves. Most tortoises retreat into a safe cave to spend the night away from predators.
My tort is always staying like that sometimes it turns back and look to everywhere. But i rarely saw it to goes for food, im always taking it out and give cucumber etc. Is it healthy i mean normal?