I've had this problem a few times where I whip out a place full of Raiders top to bottom (and it says I cleared it) but then randomly I will hear Raider voices that are like "come out and fight" and other taunts and I will search the entire place to no avail of finding them.

Sounds... er kinky....

This happens to me too. If I'm clearing out a raider camp for one of my settlements, it'll say "raiders cleared, report back to __________" and I'll still hear a raider or two around somewhere. I'll keep looking, and eventually find them. I think this is just some kind of glitch.

That's the thing, there voices will be coming from inside the room and when I find the source they are never there.