Treasure can be found in the most unlikely of places.
To throw a challenge into the mix, "boxes" are not always the place to find things. In the magical realms of beasts and dungeons, treasure can be found anywhere. From being presented on plinth, encased in some odd material, or surrounded by some obviously deadly trap...
In the centre of the room you can see a stone jutting out of the floor, and resting on top is the most beautiful pair of gauntlets you have ever seen. However, as you move into the room, you notice the wreckage of previous adventurers; their mangled skeletons strewn about the room. Clearly this treasure is protected by something...
Or, on the complete opposite end of the scale, an ogre that has set up residence in an abandoned dungeon has hoarded some impressive treasures that it collected from previous adventurers.
The Ogre stumbles backwards, the realisation that it has actually been defeated in battle finally sinking into its thick skull, and it crashes backwards through a stone wall. As the dust settles, the rock and bricks tumbling over the corpse hit something metallic. On closer inspection, you realise there is weaponry, and armour, strewn about, somewhat carelessly. It seems the beast collected the more impressive items it came across in its lifetime...
Treasure doesn't need to be "stashed away", or stored in any containers. It can simply be in a location that is unnoticeable until the time is right.
That said, the most likely of places can often even hold deadly surprises...
You don't need to "abandon" the boxes. As you mentioned, the players often get excited whenever a "box" is mentioned. You can use this to your advantage, and drop a nasty surprise on them, in the form of a Mimic. In my experience of playing D&D, my regular DM will hide a mimic in places that we would least expect it, and jump us when we least expect it. An odd door, seemingly out of place or surprisingly ornate, or overly plain compared to the rest of the dungeon. A wheelbarrow on a service elevator into an exaction site. Or a box, in a conspicuously obvious position, that immediately catches the attention of the whole party.
The box could even be a Red Herring. It contains nothing. You can use this reaction from your party to throw a spanner in the works. Currently, they expect the "box" to be important, because so far, it usually is. What they don't expect is anything else.
Trapping your "boxes" occasionally can throw the party out of their normal reaction. Whenever a "box" is mentioned, it will instead incite a feeling of uncertainty. Is it a box? Is it a trap? Is it a mimic? They won't know until they find out.
It is important to use this sparingly however. Don't make it a 1 in 3 chance. Just every now and then, once in a blue moon, just enough to make them do a double take.
"Woah hang on... remember that one time in the dungeon where it turned out to be a mimic? Let's be careful."