Someone stole the body!

[Wyrd of Gawd]

Also, I should also say we don't know anything about Joseph of Arimathea except the one reference to him in the account that labels him as a "secret follower". So it could actually be that he was an associate of the family of Jesus - perhaps a close relative or family friend, and that over the course of the next two decades with the story of the crucifixion of Jesus told and re-told the early Christians didn't know who he was and incorrectly concluded/assumed that he was a follower of Jesus.

Old Joe grabbed the body and ate it because he believed Jesus when Jesus said that he would gain eternal life by eating his flesh and drinking his blood.