Ravens store surplus food in scattered caches and routinely pilfer each other's. A caching raven that has been watched by a competitor will often retrieve and relocate the item once the observer has gone, and will cache behind visual barriers when a rival is present. This much is well established from captive and field observation.
The sharper test used a hidden setup: a room with a small peephole into an adjacent room. Ravens that had learned what a peephole allows behaved as though observed when the hole was open and playback of raven sounds came from beyond it, hurrying their caching, even though no bird could be seen. When the peephole was closed, the same sounds produced no such change.
That result is taken by many as evidence ravens can generalise from their own experience of seeing to the possibility of being seen. It is not proof of understanding another mind, and simpler explanations built on learned associations between openings and risk have not been ruled out.