本句，用 hiding 或 hidden 都正确：

Rebecca has found a chocolate egg hiding under a leafy bush.  

Rebecca has found a chocolate egg hidden under a leafy bush. 

而且，划线部分作宾补或定语，都可以。 

Rebecca has found a chocolate egg in the branches of a tree. 

划线部分作宾补或地点状语，都可以。在地面上看见的话，就是“宾补”；在树上看见的话，就是“地点状语”。