Air carrier Captain reported the undocumented air transport of Hazmat cargo.

Narrative

While the ZZZ ground crew was unloading the plane; they accessed the can in the first position. They noticed there was a box in the can that would be considered accessible dangerous goods. ZZZ was not expecting there to be hazardous freight in the can and the paperwork ZZZ1 gave us listed no accessible dangerous goods were on the plane. Cause: Someone put a box in the can without making sure it was properly documented.