Hi John,
Beside the great points mentioned before these suggestions might help you.
- I ones saw a few inserted casino / gambling URL’s pop up in a Google Analytics reports. Might be worth your time to check if there are any new URL’s created.
- Run some security scans on your site to find any vulnerabilities that might have caused the hack.
- Since you didn’t find it in your database, you could check for any hard coded links with the search through files option in Notepad. If the link is hard coded you should be able to find it.
- If you are using a CMS like Wordpress, check if all your plugins and themes are up to date. Poor maintenance could have caused the security breach.
- If your website is hacked, could it be that the hacker changed links to prevent an easy discovery? Might extent your search to more generic spam terms.
As Dirk said, don’t wait too long solving this issue. If you are depending on your SEO and you can’t fix this issue within days I would rather hire an expert then wait a few weeks.
Goodluck!