|The metal covers a small hole in the wall. It bends inwardly easily, but does not bend the other way.
A DC 10 Wisdom (Perception) check detects the presence of a small pile of something shiny and gold in the back of the hole.
A DC 12 Intelligence (Investigation) check reveals that the space between the walls and the metal is too small for medium or larger sized creatures.
A medium or larger sized creature can squeeze through the opening, the metal bends out of the way easily. This creature will discover a small pile of gold coins. However, once the creature tries to leave, the opening is too small.
In order to leave unharmed a creature must succeed at a DC 21 Dexterity (Acrobatics) check. Failure results in 1d4 slashing damage, and the option to push through with a DC 12 Strength check. If the strength check succeeds, the creature escapes but takes an additional 2d4 slashing damage, while on failure, the creature is trapped but still takes an additional 1d4 slashing damage.
The metal can be pulled off from the outside with a DC 14 Dexterity check, but requires some kind of leverage tool, such as a claw hammer or a crowbar.