>>/41143/
> Is that really a prison?

Sure.
> Without noticing and knowing the difference between being in the prison and not it would lose the meaning.

For you, and other prisoners. I presume that was the point: you won't even think about getting out if don't know you are in. And obviously Sweden implies that not everyone is a prisoner! So there are still those who can tell the difference, the jailers themselves for one.