It took some time, but this unwanted Google-script injection is the result of a value in SiteSettings.ascx.resx
<data name="Advertising.Text" xml:space="preserve">
<value><script type="text/javascript"><!--
google_ad_client = "pub-9770992166002654";
google_ad_output = "textlink";
google_ad_format = "ref_text";
google_cpa_choice = "CAAQ3KfzygIaCNJbQ5r5YiDXKKjntoQBMAA";
google_ad_channel = "";
//--></script>
<script type="text/javascript"
src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
</script></value>
</data>
By removing the above code and replacing it with something useless like
<data name="Advertising.Text" xml:space="preserve">
<value>Don’t waste your life on advertising!</value>
</data>
the problem are gone (until next upgrade).