Try a few other scanners:
https://wordpress.org/plugins/wordfence/
http://wordpress.org/plugins/gotmls/
Also, you can upload a backup of your site to scan here:
https://www.virustotal.com/
Keep in mind that even if you locate and remove the iframe, the question will remain of how it has got there in the first place .... This typically means that your site security has been comprised, and you have two choices:
a) restore your site from a good known backup (and of course change all passwords (WP dashboard/Cpanel/database) and update your salt keys).
b) Work through the advice in the Wordpress codex here.
Once resolved, you should consider implementing the advice in the Hardening Wordpress Codex.
Good luck!