A mystery box (or puzzle box) is a codelocked box that contains a prize. A clue to the code would either be near the box, or would hint at where the box was located.

This box (built in an art gallery in November 2022) was placed under a sign that read MMXXII. To the right is the Government flag (which was displayed as one of the paintings, irrelevant to the puzzle).
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3070032497

The solution was that the wooden sign contained the code in Roman numerals. The code was 2022. The box contained food as a prize. Players solved the box by destroying it, and when it was replaced it was destroyed again.

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