For those of you wondering what happened in Greece with the afghan immigrant:
The Greek prisons in the last few years have gotten stacked with criminals and way over capacity. The government passed a controversial law aiming in reducing the strain they are under.
What this law does is reduce the maximum sentence time and give you the possibility to be released under parole if you have served 1/10 1/5 etc of your sentence depending on the case. Basically it says "get out we need to make room for more". This dude also claimed he was an unaccompanied minor which gave him further benefits and reductions of his sentence under the same law. So they released him under parole and he left.
Btw, I don't know if this happens in other countries but if you are a prisoner here and behave well there is a chance you can also take leave from prison. Even if you are a murderer/terrorist/mob boss in for life, you can possibly at some point earn the right to just leave prison to go see your family or whatever for a few days, as long as you promise you will return. A lot of people just walk out those jail front gates and never come back