i had a same issues on my few sites,
I installed Wordfence Security and scanned the files, it showed all secure without scanning the theme and plugin folder, so i enabled the Wordfence security to scan the theme and plugins from settings and saved, then i re-scanned it, it showed some hack files on themes folder (wp-content/themes/theme-name). the hack is not in header or php files, it was in the theme folder file name (social.png).
There were ten files from (social.png, social0.png to social9.png), and i deleted the file and submitted to google webmaster tools, fetch as google.
and checked it, and there were no casino links on my header.
Please check if it works for you…
i had a same issues on my few sites,
I installed Wordfence Security and scan the files, the hack is not in header or php files, it was in the theme folder file name (social.png).
There were ten files from (social.png, social0.png to social9.png), and i deleted the file and submitted to google webmaster tools, fetch as google.
and checked it, and there were no casino links on my header.
