I think it would be very unlikely (although certainly not impossible), for the lunar astronauts to have stumbled on a meteorite. My guess is that it is at least several orders of magnitude more likely to find a metorite on Mars than on the Moon. First, Mars sits right next to the asteroid belt. It probably doesn't take much to nudge an asteroid into a Mars crossing orbit. Second, Mars doesn't sit right next to a big ol' gravity sink, like the Moon sits next to Earth.