I think it may be contemporary to “Let the right in” (the 2008 original Sweedish one, not the USA remake), and probably is Scandinavian as well, though it could be German. Anyway, I’m pretty sure it’s from Europe.

I can only remember that there was some sort of hidden, parallel apartment. Someone starts hearing some noises (someone crying, or having sex) from the neighbours, and discovers hidden corridors and rooms. Doors and walls are broken to discover new spaces.

The tone of the movie is dark, misterious. It’s a thriller, with some horror elements as well.

The title could have some of this words: “hidden”, “next door” or “neighbour”.

I’ve forgot almost everything about it, but I know I enjoyed it very much.