A quick post to let you know that after 90 days Marketo will automatically archive several activity log entries. Once archived, the activity will still be visible on the Activity Log tab on the individual lead, but it will not be visible in Smart Lists or Reports.
Here you have the detail:
The 90 day Automatic Data Clean Up has been put in force to improve performance, especially the speed Smart Lists and Campaigns run.
Here you will find more details on this: Understand How Marketo Archives Inactive Anonymous Leads