I am betting it burned up as its trajectory when they jettisoned it was headed toward earth. I think it was Apollo 12’s SV-B that was lost and found and maybe lost again, could be one of those from a different mission.
I looked it up. They sent the Apollo 13 service module on a trajectory to impact the moon. The Apollo 12 S-IVB is still floating around up there - somewhere.
https://en.m.wikipedia.org/wiki/S-IVB