On top of Sanford's answer, leads added to Marketo are not sync'ed to SFDC without a specific action being triggered. Some are explicit flow steps, some are implicit, by-products of other flow step or actions not apparently linked to SFDC:
Explicit SFDC related flow steps
- Sync to SFDC
- Add to SFDC campaign
Implicit:
- running a "Change Program Status" flow step, if the program is sync'ed with an SFDC campaign
- Importing a lead in a program, if the program is sync'ed with an SFDC campaign
- When a lead fills out a form located in a program, if the program is sync'ed with an SFDC campaign
Now, best practice is that you should always import leads into a static list located in a program, to ensure traceability. Read Why you should always use a static list to import persons in a program
-Greg