Welcome to the Q&A Forum

Whoa, this is a weird one. I saw that you posted this on Google's forums as well, and they suggested that this might be the Japanese keyword hack. Did you look into that? If that's not it, did you try loading the URLs that are showing up on the Wayback Machine? It's possible that someone who owned this site before your client created these pages. Either way, the answer is to double check that your 404 pages really are 404ing. If that doesn't remove them from the index fast enough, you can actually create all of those pages, with a noindex tag, add them all to a sitemap, and submit them to Google. But the 404ing is really your long term solution. Good luck!