Originally Posted by Duboris
"As you detect the room for magic, you sense only your own belongings, the flight-capable wings on the back of the knight handing the box to you, his weapons, all the weapons of the sergeants around you and that's all you sense. For some reason that currently escapes you, however, you are not capable of detecting any trace of magic within the box, as your detection cannot pierce it's adamantium walls."
See, your mistake was including the parts I bolded. You can't straight-up tell your players that you're hiding something and expect them to not want to know what that something is. If you had simply not
listed the box among the objects that do
detect as magic, you might have gotten by invoking only mild curiosity.