In the developer toolbar, there’s an h1 with .headline__primary class that doesn’t ‘feel’ right to me. The negative right margin might be creating extra width.
What you ultimately might have to do though is hide all the main parent elements (the ones that are siblings/in the same level of the DOM), barring one, in turn, to see which holds the offending element;
#fancybox-tmp, #fancybox-loading, #fancybox-overlay, #fancybox-wrap { display:none }
This would leave only #page visible.
Check in iPhone. If the margin is still there, work through the main sibling elements of #page and do the same as before;
#page header, #page article, #page footer { display:none }
This would leave the only div inside #page visible.
Check in iPhone again. If the margin is now gone, you know that header, article, or footer will be, or contain, the culprit.
Keep narrowing down like that and you’ll eventually find where the problem element lies.
