The LastOutput line on command blocks sometimes does not show up. Some commands make the line disappear, some make it appear. When the output is turned off by switching O to X, the output line shows (however of course without any output).

Test:

1. Give command block
2. Place command block
3. Look in command block to see the LastOutput line is not there
4. Input

/testfor @p

and run to see now the line appears
5. Input

/say @p

and run to see now the line is gone