Well, supposedly a Mars-sized body slammed into the early Earth and reformed the Earth with the Moon. So that's one place a Mars-sized planet could have gone. Of course it's also possible that there just wasn't enough material in that slot to clump together and form a planet.
The moon is heavily cratered, and so is Mercury, and it looks like earth is, too, although erosion and other action has obliterated a lot of the obvious earth craters. There was a lot of big stuff out there at one point.