Scroll back to comments around mid-August where I commented on same issue and a couple ways to deal with it.
You can combine this (superb) addon with Handynotes.NPCScan will tell you that the NPC is definitely there. Go to your main map and Handynotes will show you where the NPC normally spawns. Go there and you will, mostly, find him. I say "mostly" because a few NPCs roam, and they can be a little trickier to find. But this works for 90%+ of rares.
Love the addon-thank you for all the work on it!
My question is, is there a way to set up the tool so that even tho a spawn has been detected for an achievement, it will still alert if the spawn has a special drop that has not been collected, toys and such, without having to disable for all spawns.