SFDC without an Account

We experienced this one this week due to some changes. Really had us stumped for awhile. 

It's possible to create a contact with out an account, in SFDC land they call it a private contact.

Most SFDC orgs will have the Account lookup field as required on the contact page layout, but this can be unchecked at the page layout level allowing users to create contact records without selecting an account. Other ways may be via list imports into SFDC.

Contacts not associated with accounts are private and cannot be viewed by other users or included in reports. Only the owner of the contact and administrators can view it. Contact sharing rules do not apply to private contacts.Therefore unless your Marketo user happens to be the owner of the private record or it's profile has the modify all data/modify all contacts permissions then I would not expect Marketo to be able to see it.