How does Marketo unanonymize leads synced from Salesforce?

I have a lead who Marketo was able to bring in anonymous activity for prior to their becoming a person via salesforce sync. How was Marketo able to do this when this person did not have an email to associate with the SFDC lead coming in?

I understand that Marketo cookies leads and when they fill out a Marketo form and become a person Marketo can match their cookied, pre-form fill behavior with this new email and person that has been created. In this case, however, I cannot see what Marketo is using to make the association.

Any thoughts?