MD-11
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Situational Awareness
Deviation / Discrepancy - Procedural Hazardous Material Violation; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Overcame Equipment Problem; Flight Crew Became Reoriented
Human Factors
Blocked out of ZZZ 53 minutes late due to late inbound aircraft. Another MD11 was loading up beside us (Aircraft Y); also bound for ZZZ1. The flight was normal; until I received an ACARS message from our Dispatcher that there was undeclared hazardous material onboard in position 3L. I immediately made a SAT phone call to Dispatcher. He said they did not know anything other than it was Class 9. He had spoken to the Duty Officer. My excellent (and probationary) First Officer quickly brought up the relevant section of the FOM; 10.25; under 'DG Manifest Modification.' After consulting with the Dispatcher and the First Office; I decided that; since there were no physical signs such as smells or smoke that would call for a divert; and that there was no other Hazmat onboard that may create a separation issue; we would continue to ZZZ1 if the Duty Officer concurred. We would modify the DG Manifest in-flight. I notified the Dispatcher of this. He informed me later that the Duty Officer concurred with the decision to continue. In an abundance of caution we donned the CO2 detectors; as we were not sure what Class 9 Hazmat we were carrying. [We] received another ACARS. We were informed that the Hazmat was some sort of 'safety devices.'  We were in contact frequently with the Dispatcher to see if there was any new information; but there was not. Landed uneventfully in ZZZ1.
At cruise we received an ACARS message from [Dispatch] that the ramp in ZZZ had notified [Dispatch] there was an unmanifested IDG 342 kg Class 9 Hazmat in position 3L. Captain SAT phoned [Dispatch] to talk about what was going on. ZZZ didn't know exactly what it was but it was Class 9 'safety equipment'. Talking it over with [Dispatch]; Duty Officer; and ourselves we elected to continue. We were able to update the DG manifest to reflect 'class 9 safety equipment.' We sent subsequent messages trying to find more information about the unmanifested cargo but [Dispatch] was unable to get ahold of ZZZ to further identify the cargo. We were able to put as much information as we could on the updated manifest. As a precaution we pulled out the CO2 detectors in case dry ice was involved. The rest of the flight went without incident and we were instructed to file ASAP and FSR reports. We also have photos of all the documents in case those are in need of further review. We were able to comply with FOM page 10-13 regarding updating a DG manifest after block out.  ZZZ Ramp did not list the IDG located in position 3L on the DG manifest. The DG manifest reflected no dangerous goods on board.
MD-11 flight crew reported ACARS notification of unmanifested Dangerous Goods on board. At departure; received no Dangerous Goods required documentation.