Hi Michael,
Any updates that are synched via the background sync (normally every 5 mins) will always start off where they last finished.
So imagine you has a successful sync on Friday at 3pm, then a sync failure at 3.06pm on Friday due to an invalid password, you don't realise there is a problem until Monday and you change the password on Monday at 9am.Vverything that failed between Friday at 3.06 and Monday at 9am will be processed.
What you will have to catch up on are any SFDC flow action in your Marketo campaigns that would have been processed in that down time. Any Sync Lead to SFDC actions, create task, add to SFDC campaign etc.
The best option would be to clone you campaign to create a batch campaign that uses the filter equivalent of your trigger and involve an time constraint.
Password expiration periods can be set in SFDC so that the likelihood of this occurring is lessened.
http://community.marketo.com/MarketoArticle?id=kA050000000L3ddCACYou might want to vote for or leave feedback on the following idea:
http://community.marketo.com/MarketoIdeaDetail?id=08750000000HciFAASCathal