In order to see it we'd need to know exactly where to look. In order to know where to look we need to see it.

If the orbit is eccentric enough then when it's closest to the sun it will be much easier to spot. But that could be centuries from now.

With deep space radio telescopes I think we'd have a good chance of finding any missing planets, could use active radar to scan large sections of the sky.

That also could take centuries though.