Before the later missions when the upper stages were crashed into the moon as targets for the seismic instruments, they flew past the moon and on into either high earth orbits or solar orbits. The Apollo 12 stage that was recently rediscovered had done the latter. After years of orbiting the sun, it returned to the vicinity of earth and was recaptured, temporarily, into a high earth orbit. The tracking data fitted this scenario.
http://neo.jpl.nasa.gov/j002e3a.html