In version 2.3.11 a non-editing teacher was able to view forum posts of participants in separate groups via
Courses > [course] > Participants > [user] > Forum Posts > Posts". This was useful when reviewing participant activity for course completion purposes after the course is closed to participants.
In moodle 2.6.2 the forum posts tab remains listed in the navigation but is greyed out and, on clicking it any student user (participant), the posts are not visible and instead an error message appears saying:
More information about this error (linking to http://docs.moodle.org/26/en/error/forum/notenrolled).
The posts of higher roles (non-editing teachers) are visible.