Unavoidable mirror modifier 2.76b   using the default cube I subdivide edges with 10 cuts. Unselect all. Rotate the cube and box select only 1 side. P to separate by selection. 
Now in sculpt mode, all sculpting is mirrored about the center of the plane I separated.
Sculpting can be made symmetrical in the toolbar.
Hit T to show it if it's hidden.