Pat, my understanding is that this occurs any time "images are turned on"... either by manual action (clicking and approving images), or by user settings in the individual's email client/browser that may auto-turn-on images.
If that's not correct, I'll happily read what others add to this thread!
Yes, because if the lead never downloads images nor clicks a link, they haven't (from the tracking server's perspective) engaged with the email in any way that's different from someone who deletes the mail immediately.
As far as I know, the only technical way possible to know whether an email has been opened is by reading impressions of an image that usually get included as a hidden asset of the email HTML version. Obviously, if a link within the email message is clicked, the message should also be considered opened.
If the preview can trigger any of these conditions, then it is possible that the message gets counted as opened. All different email clients can behave and treat email messages different, so there isn't really a single answer that can be correct.
Marketo implements an image at the bottom of each HTML message in the format of "YOURMARKETODOMAIN.com/trk?t=1&mid=NzQ4LU1CSy00NTc6NDM2NjowOjA6MDoyNDg1OjA6MDpyYWZhZWwuc2FudG9uaUBjaXRyaXguY29t" width="1" height="1" border="0" ] to keep track of opens.
Keep in mind that as opposed to SENT, DELIVERED, and BOUNCED statuses, the open state of a message is not reported back from the recipient server to the sender server. That is why a hidden image is used to track opens and why the pure TEXT part of a multi-part email can not track opens.
It seems like we were all typing responses almost at the same time. I thought I was going to be the first one responding. Didn't mean to downplay anyone's answer. We are pretty much all saying the same thing in our own little way.