Has anyone heard of Salesforce tracking an email click through, but not an open? We have some odd examples in SFDC of some Marketo emails not showing as opened but are recording a CT when that happens. Is this a pixel issue?
Marketo only syncs one activity of each type per day per lead, so using SFDC as a guide isn't really useful.
On the Marketo side, you absolutely will not get an Opened Email if the person has images off.