The man enters, walks past Eve hiding behind a plant, and steps into the pool of blood on the floor.

Is it OK to write '...Eve hiding...' instead of '...Eve who's hiding...' if it's already known where Eve is hiding?

Would you put a comma after 'Eve'?

anonymousalready known where Eve is hiding

In that case it's

The man walks past Eve, (hidden) behind the plant, and steps into ....