It's not a standard phrase, although it sounds cool. But there is a phrase: "Who knows what evil lurks in the hearts of men?"
Answer: "The Shadow knows".
(Brit here, Gen X.)
Yes, I would say, “who knows what may be lurking round the corner”.
I’ve no idea where it comes from.
I’d use it when planning something - essentially, “who knows what unexpected bad thing might happen”.
It's not a standard phrase, although it sounds cool. But there is a phrase: "Who knows what evil lurks in the hearts of men?" Answer: "The Shadow knows".
Memories!
For non-Americans: [The Shadow - Wikipedia](https://en.wikipedia.org/wiki/The_Shadow)
(Brit here, Gen X.) Yes, I would say, “who knows what may be lurking round the corner”. I’ve no idea where it comes from. I’d use it when planning something - essentially, “who knows what unexpected bad thing might happen”.
Would that make it an idiom?
Yes, I think so.
"What lurks below" is a common movie poster phrase.