If there was anything of any size there, we would be able to measure its effect on the rest of the planetary orbits. Also, it would not be in a stable orbit, a

If there was anything of any size there, we would be able to measure its effect on the rest of the planetary orbits. Also, it would not be in a stable orbit, and would eventually either get ejected from the solar system, move into a different, stable orbit, or intersect with us (as Theia presumably did).