Even if an activity is marked as private (the 'private - do not track usage for this activity' checkbox in the 'other' tab of the activity settings dialog), files opened in that activity appear in the 'recent documents'.

Steps to reproduce:
1. Create a new activity and mark it as private.
2. Open a file in the activity.
3. Switch to another activity.
4. In krunner, type 'recent'.

Observed result:
The file which was opened in the activity marked 'private' is visible in the list of recent documents.

Expected result:
Files opened in private activities should not be tracked by the 'recent documents' feature.