Today our idiom is, " a hole in the wall ".
We use this when describing a small, un-noticeable place (usually a restaurant).
A. Where did you go eat last night?
B. We found this little hole in the wall about 10 minutes from here.They served the best Mexican food I've tasted in a long time.The prices were reasonable too.
My office is just a little hole in the wall in downtown Tokyo. I can't afford anything else.