You can leave a note about a patient that is visible only to practice admins. Navigate to the patient profile and click on the notes section. 

Admins can also delete a note. Navigate to the note and click 'delete' on the bottom right corner of the page

You can flag a note by clicking the flag button to the right of the note (see image above). This will flag the patient as a whole- the blue flag icon will show to the right of their name and will be visible from the main patient page. To remove the flag, simply press "unflag".