Air Carrier Ramp Crew Lead reported being notified notified of a cargo priority package overfly. Priority package was inadvertently left onboard primarily because package was improperly packaged and missing Hazmat labeled.

Narrative

My crew unloaded Flight ABCD at gate XX at XA:14pm 1lb priority shipment didn't come off the flight from [section] X. My crew emptied the whole X belly and did not notice any priority Shipment. there was 2 Crews unloading this Flight. We did the front and [section] Y. Person A and Crew unloaded [section] Z. We did the inbound only. Person B and Person C unloaded the X Belly. We also had emptied animals from A and no priority shipment.Today I came to work and was notified I was [assigned] to a meeting about the priority shipment overfly.I was told it was a 1Lb letter. I didn't even know what it looked like. I was told there was blood in the letter but seen no such letter on the plane.If this letter that had blood in it it should have had a Biohazardous sticker on it and put in a priority shipment bag. I can't believe this was on my flight since nobody on the in ground crew or the out bound crew [saw] the letter. Was there pictures taken of the priority shipment taken off in ZZZ1? In the future the 1lb envelope should be packaged in a more visible package bag or more noticeable box.