This question just came to me and I'm not sure if this ever happened in tv series, movies or novels but was there ever a time that a watcher never knew about their immortality then one day they get killed and discover that they are back to life. Upon that happening, did the watchers kick them out or tried to go after them like Horton's group did or were they fortunate enough to remain with the watchers?
No. If a watcher were to become Immortal and the other watchers found out the newly Immortal Watcher would be kicked out of the watchers. In fact there was an episode in the TV show where the watchers discovered that Adam, a watcher himself, was actually Methos the oldest living immortal(he was over 1000 years old). He was removed from his position immediately upon discovery.