Is the field visible to the Marketo User and/or does it exist in Marketo and shown to have synced in the past under Field Management?
The short answer is "test it" on 1 record. It will likely not sync the record or data.
- if it does sync, then you need to adjust User settings in SFDC.
- could break sync for a short period, but risky.
Oh also, Marketo looks at the sysmodtimestamp that SFDC has on the SFDC field itself. So if you update a field, Marketo sees the update was recent and tries to sync the data.
Thanks Josh. That's what I suspected. The value in the new field doesn't sync to Marketo because the Marketo Sync user doesn't have visibility to the field; however, the sysmodtimestamp change still triggers the record to sync.