[powerbroker]

This seems the most straightforward of explanations. I know that a planet tends to loose its heat (and visibility in infrared) quickly after formation -- and might be hard to spot as a rogue planet.

But, the Zwart explanation seems to suggest that the pairs are still potentially drawing stuff in, and might not be the final form that the planet-pairs will take once they exist (or consume) the nebulae. Nothing seems to be stopping these pairs from evolving into stars themselves (aren't binaries far more common than solitary suns, like our own?).