Well I'm going to maybe blow your mind some more here because those show up I believe as a lead merge activity. We then take the targetid of the merge and go back with non-bulk API for any leads that have been made known and get their activities. It's not great.
Also speaking of merge, we also need to keep activities sorted and mapped to the proper leads for aggregate reporting, and we need to keep leads sorted, so we have to look at all delete lead and merge lead activities. If we get a delete lead, we mark the lead as deleted. If we get an n-way merge for leads and they were merged into another person, we then change the leadid for all old activities for merged leads, and mark the old leads as merged/deleted.
This also means we need to regenerate summaries related to lead/activity counts since the activities can be remapped to different leads later in merges.
There are a lot of gotchas if you're trying to keep data synced with marketo.