Air carrier Lead Technician reported undeclared HAZMAT went unnoticed and was transported on two separate flights as the reporter was not informed that the box contained anything hazardous and there were no markings on the box.

Narrative

The event happened on Day 0. The event happened on Aircraft X from ZZZ - ZZZ1 and Aircraft Y from ZZZ1 to ZZZ2. The event was a hazardous part was shipped as an AOG undeclared on two Company flights. The event was brought to my attention at XA:00 - XA:30 PM; Day 3; by Company management. The part shipped was an airplane rotable which could have had residual fuel in it. The part was inside a 12x12x12 box.Cause: An employee dropped off a hazardous part at the AOG desk stating it needed to go onto a flight there was no hazardous markings or stickers on the box; and it was not disclosed to me that it was hazardous. As close as I can recall I shipped it without checking the inside of the box as employees are supposed to declare when they bring hazardous parts to the AOG desk. I did not remove the paperwork from the slip as I had all items needed to ship the box without removing it. The paperwork that I viewed was never pulled out of the box and the box was sealed. I had all the required information to ship the item; and nothing told me it was hazardous.Suggestions: There are several issues that could have helped to prevent this. First when a part is issued by a hanger; and they ship it themselves assigning their own air waybill/waybill unless their password is up-to-date you are unable to print original shipping document. When you are unable to print the original shipping document the print screen function does not display the hazardous material label or logo. The shipping document does not display hazardous materials in an obvious way it is at the bottom of the document on the second page and is very similar to the way Electrostatic Discharge Sensitive (ESDS) or shelf life are displayed on a shipping document and is easily overlooked. The easiest way to avoid this is to prevent anyone who is not physically shipping the part out to not print shipping labels or attach waybills. This means outside Shipping; no one else attaches a waybill/air waybill to anything. Anyone who issues a part should clearly display hazardous materials logos as is standard procedure on all hazardous parts boxes. The design for the shipping document should have the hazardous material label clearly displayed in large letters at the top on the first page and be completely different from all other designs. The final step should be that I should put every single number into HAZMAT system to verify that they are not hazardous and open every single box to verify the paperwork is correct. If the issuing document had been displayed on the outside of the box it would have been unavoidable. The part would not have been missed.