If you want to get all techy. The way i look at is. Scientists did not have the technolgy we have today and so if they were not searching an area of space that was thought had nothing in it at the time when they first looked they thought there was nothing there and were not looking to find anything they wanted to put there foucs on the stuff they new they would find. Not to mention space observers go back all the way to the dawn of time. They only recently started to put objects into space in the late mid 20th centery if i remember correctly. At this point in time scientists are looking for anything they can find that would be a potential hazard to the Human race (Aliens, Astroids, planet collisions, Black Hole, EXt...........)
( The main point i am trying to make is if recently they found it might be because it finally came in to view. Because if it takes 10,000 years to orbit the sun) Chances are there not going to see it till it comes in really close.)
I mean if it takes 10k Years to orbit the sun that puts the planet what 76-86 astro light years away.
( Sorry about the spelling not the greatest at it