The drop happened site wide, in all sections of the site, from all geo-locations, from all sources (Search engines, direct,...), from all devices (mobile,desktop) and it happened also to our Spanish and German version.
In other words, it is not only from search engines but also direct and referral traffic that is down.
It is not seasonal because we have historic data and there were no worldwide major events this week.
One option that we need to check is that some people just don't reach our site due to technical issues. That would not be displayed in Google Analytics, as people do not get the Html.
Question: How can we know if requests are failing or taking much longer? Are there any tools out there?
If your having load speed or other issues it should show up in the webmaster section. I would also be hitting the site from proxy servers to see if the traffic is being redirected by a DNS hack. https://www.google.com/search?q=DNS+Poisoning+&sourceid=webmasterworld&num=100&sitesearch=webmasterworld.com