Here's the scenario:
Someone came to our site and filled out a form. The campaign that owns the form sent an email alert to the account owner that the form was filled out, and created a task in SFDC for the account owner to follow up with the lead. It was also SUPPOSED to add this person to an SFDC campaign.
However, the lead was already unsubscribed, therefore, imported into Marketo as unsubscribed. Per our operational campaigns, they were deleted from Marketo 24 hours later, but NOT from SFDC. I have a record of them being put back into Marketo 2 days later from a salesforce sync but still as unsubscribed.
This person is not showing up in the Salesforce campaign and there's not an active Trigger in Marketo that would have deleted or removed them. Why wouldn't they be in the Salesforce campaign?
Interesting case. There's no specific reason I can think of that they wouldn't end up in the SFDC Campaign even after being deleted in Marketo only. I'd be thinking they were never in there.
Any chance the SFDC Campaign synced with a Marketo Program, which disables the direct Add? Can you reproduce the behavior with a brand new lead (seems like you should be able to repro this whole chain of events)?