While scrolling to the least popular tags, I came across the missing-scene tag, with no apparent questions. Was this tag popular at one point or why does it exist?
During a discussion in chat about the possibility of a scene-identification tag, I noticed that there was a missing-scene tag with a single question. I then edited that question to remove the tag. You just happened to notice it at precisely the time when it no longer existed but was still shown in the system due to caching. As of now, it no longer appears in the tags list.