System Information
Tested in Windows 10, 7 and Mac OS 10.6.8

Blender Version
Broken: 2.77

Short description of error
Subsurf modifier does not trigger is_updated or is_updated_data on the active_object when changing render_levels.

Exact steps for others to reproduce the error
Open default scene.
Add subsurf modifier to cube.
Run the script below.
Change the render_levels value for the subsurf.
Check output in the System Console.

import bpy

def track(scene):
    if bpy.context.active_object.is_updated:
        print ("Object is updated")
        
    if bpy.context.active_object.is_updated_data:
        print ("Object Data is updated")

bpy.app.handlers.scene_update_post.append(track)