You can see them here: untagged
I'll clarify the process here, since this question got bumped and the full explanation isn't in an answer yet.
There is a periodic process that will remove any tag that only has one instance and has lasted for 6 months. If there are other tags on the question, it will just remove the tag. If it is the last tag on a question, though, that question will be marked with untagged. The former is actually more dangerous, because it is more difficult to track and notice. But this is the origin of the tag.
You can prevent this easiest by simply asking another question that needs the same tag. As well, this is also a good reason that, when introducing a new tag that applies to old questions, you should find at least two candidates. As long as there are at least two instances, the automated process will not remove the tag from existing questions.