Sometimes, you may receive an auto response from an email that has been sent through a batch campaign. The auto response email will not show the lead's email address instead it does contain the HTML of the email along with the Marketo decorated link.
Is there a way to decipher this to determine which lead record this corresponds to?
You could try opening a new browser window and either clear your cookies first or use the browser's private browsing feature (makes a cookie sandbox) so you don't confuse Marketo and tie that person's lead to you by accident.
Anyways, in your cookie-free browser, click the link and then go to a form that Marketo prefills for you and voila you can see the email address that was originally targeted.