If you want to open your eyes and chest, you must cast a spell when you leave your eyes and chest, which means you can't be seen by him. For example, you can walk behind the closet and open it.

In the "Secret of the Forbidden Area" task, the first eye treasure chest you will be able to open is located in the restricted area of Hogwarts Library.

If you complete the main task "The Secret of Forbidden Area", you will unlock a magic spell to make you invisible and avoid being discovered by others.