My first reaction was "it shone for 14 days and only Korean astronomers noticed it?" Then I googled for that nova and eventually found this interesting article https://academic.oup.com/astrogeo/article/47/1/1.29/258488/W...

It seems that Koreans were particularly keen at looking for novae. Cloudy skys, lack of fast global communications, the small number of people looking for faint new stars could explain why nobody else noticed it. For example, about the supernova of 1604 which was visible during the day:

> "On the 30th [of September], the sudden breaking of the clouds afforded one of Kepler's friends an opportunity of having a very short view of it… On the 2nd, 3rd, 4th and 6th of October, it was seen by several persons in different places. On account of cloudy weather at Prague, where Kepler resided, he did not see it until the 8th of that month."

If that's happened to a supernova, I can imagine a nova could slip away unnoticed.