You see email clicks in a lead's activity history but no corresponding web page visits to the Munchkin-tracked page. The clicks happen immediately upon delivery of the email, or sometimes even before the delivery is logged.
This is usually caused by email security software on the receiving email server. The security software tests the links to make sure they are not malicious and this causes Marketo to log a click activity for the email. Because the security software does not actually open the web page in the browser, there is no web page visit logged.
Marketo has a feature that embeds a hidden link in the email that should only be visible to the security software. When this link is tested by the security software, Marketo notes the source of the click and ignores all clicks from that source. While this may not catch all test clicks from the security software, it catches most of them. Contact Marketo support to confirm that this feature is enabled for your instance.
Your emails are more likely to be link tested if your sender reputation is low. For more information on link testing, see the following documents.