After successfully sending Jesus to the cross, the members of the Sanhedrin feared that His disciples would sneak in and steal the body. They asked for and received soldiers to guard the tomb. Although they didn't believe such a thing could happen, they took Jesus's claim that He would rise again in three days in a literal sense. But His disciples, on the other hand, don't seem to have shared the same conviction, despite Christ's promise.
On the day of the resurrection, two of them set off toward the village of Emmaus and away from the tomb (Luke 24:13). Instead of expressing hope that Jesus had risen or would rise, they showed their disappointment that He had not fulfilled their hopes of a present Messianic kingdom (Luke 24:21). Even news of His missing body simply confused them. Jesus had given them plenty of warning about His impending death and—to a lesser extent—His resurrection. But it wasn't a bodily resurrection they expected.
Given their background and beliefs, the disciples had no real motive to steal the body. They might have expected a spiritual resurrection—that is, a continuation of His work and ministry or His vindication after death. At most, they might have believed Him to have meant a ghostly appearance, since most Jews believed the soul continued after death. However, they had no real foundation for expecting the body to physically return to life and, thus, no need to do what the leaders in Israel feared.
In fact, the disciples' radical change concerning their belief about the resurrection—not to mention the transformation from fearful hiding to bold proclamation—proves that, at the least, they themselves became convinced that Jesus rose from the dead. Although faced with scorn from Jews and Greeks alike, they preached not just a continuation of Jesus's work, but a Savior who lived again.
Adapted from The Life and Times of Jesus the Messiah by Alfred Edersheim (Book V, Chapter XVI).