Users with admin role assigned at the system level show up on the participants drop down list for recent activity in courses they are not enrolled in.
1. Go into a course that does not have system level administrators enrolled.
2. If the course doesn't have the recent activity block add it.
3. Click "Full report of recent activity ..." link in recent activity block
4. Click on participants drop down list. You will see site admin users that aren't in enrolled in the list..