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.
I do use Handynotes, just didn't realize I needed a separate one for BFA (kept waiting for those I had to update). Your response prompted me to check and sure enough there's a new one for BFA, so thank you!
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.