Microsoft Outlook 2007, 2010, 2013 have images "off" by default for HTML emails unless the sender is a trusted address (white-listed) or the user has otherwise specified that it is OK to display images from that particular sender. Marketo knows an email has been "opened" via small embeded image. Whether or not a recipient views the message in the preview pane or in full view, if the images are disabled Marketo cannot see that a message was opened. However, that recipient may still click a link, hence you would not see an "open" from that recipient, but you would see a "click" and web activity.
This is why I put much more emphasis on click and conversion results of A/B testing than I do sheer open rate.
Example: I just received the message below from Shutterstock. The images do not display by default, but I clicked through. They will read a click with no open activity for me for this message. This message also demonstrates why emails consisting entirely of images is a bad idea.