Not to mention that such a body, passing through the inner solar system every 300 or so years since antiquity, would be well known and chronicled. Think about how small Halley's comet is (about 6 miles diameter) and how well known and well-documented it is.
The point you have made is worthy of consideration.
Thank you for your insight.
You commented a few days ago on the visibility of objects in the solar system. I gave your comments some thought and updated my article october-5-2017.com to list the date range wherein I think that the Red Dragon may be visible.
The most important point is that the night side of the earth has to be facing the path of the Red Dragon in order to be visible to amateur astronomers. This is only true from about October, 2016 to about April, 2017. For both the six (6) months before and after that period, the day side of the earth will face the path of the Red Dragon.