Files recently opened in a private activity are visible from KRunner in a standard activity – labeled as "Recently used".

How to reproduce:
1. Create a new activity and check the option "Private – do not track usage in this activity".
2. Switch to the new private activity.
3. Create a new file ("Top secret") in any format.
4. Open the new file (from the private activity) and then close the file.
5. Switch to a standard activity
Alternatively also stop the private activity, but this is not necessary.
6. Search for the file ("Top secret") that you opened in the private activity.
KRunner suggests the file as a "recently opened document".