C208 Captain reported the undocumented transport of Hazmat material (fire extinguisher). The Captain stated no Hazmat documents were provided and that he failed to notice the fire extinguisher on board.

Narrative

On Day 0 I was flying a return flight with cargo from ZZZ to home base in ZZZ1 for my company I work for. I was flying in a Cessna Caravan 208B. On my flight was loaded hazardous materials (fire extinguishers) in a plastic container that I failed to see and identify without proper paperwork for shipping. The flight was safely completed but I failed to have the proper paperwork in order to fly the hazardous materials. It was later reported to me that someone had found the hazardous materials (fire extinguishers) and failed to see any paperwork for them. I told them that I did not know the hazardous materials were aboard the flight so there was no paperwork for them. A report was made about it and I am writing this report as well. The failure to have the proper documents and paperwork aboard the aircraft was on me for not checking to see if there were hazardous materials in the plastic containers. I will make sure that next time I do a better job of looking over all the cargo and any containers next time to make sure that I identify any and all potential hazardous materials that go into the plane that need proper paperwork and documentation when aboard the aircraft.