Nytmare
David Jose
Player: So we made up some guys, what's there to do around here?
DM: Goblins in the forest, a dungeon full of spiders, rumours of a dragon in the mountains, and an ogre fort in the hills.
Player: Hmm... I can't decide. Any rumours about magic items they might have? If one of those places holds a nice magic sword I'll head there first.
DM: ...
If there's a rumor about a sword, and the rumors are based off of anything approaching fact, that means that the story has taken a peek inside the box.
"There are stories of a hedge knight who led a party of mercenaries into the mountains to vanquish the dragon decades ago. The stories are numerous and most seem to be far more fiction than fact, but they all seem to agree on one thing. The nameless knight had a silver sword that shone like a midnight sun when he whispered to it."
That also means that, as a DM, if you've populated a treasure with a magic sword, you've taken on the responsibility to either make sure that there's still a magic sword when the players get there or come up with a reason why it's not there anymore.
There isn't some kind of magic window that lets players peek into a treasure chest to see if they want what's inside yet.