Automatic Background Updates For Plugins Even Though Disabled

I'm working on a client's wordpress site, and there's an issue with the plugins automatically updating in the background. I rollback 3 plugins, and later find them updated again.

I know that the 3.7 auto updater does not have plugin updates enabled by default, so this doesn't seem possible. To be safe I manually disabled plugin updates with this code in the wp-config file:

add_filter( 'auto_update_plugin', '__return_false' );

However they still auto update! I then installed a plugin to manage auto updates, but that didn't stop it either.

Is this a problem others are having, or am I missing something major in my theme? I did a search for any code enabling this, but found none.

Thanks!