Drupal 7 Core Module Function for node_save?
DO NOT HACK CORE

Now that's out of the way, let's continue.

The way I see it, you have a couple of options:

1) Use Rules.


  The rules module allows site administrators to define conditionally
  executed actions based on occurring events (known as reactive or ECA
  rules). It's a replacement with more features for the trigger module
  in core and the successor of the Drupal 5 workflow-ng module.


Although this will allow people to see that there is code being run.

2) Create a custom module, but in your info file make use of the hidden property:


  As of version 7.x, modules and themes may specify that they should not
  be visible on the modules page by adding hidden = TRUE. This is
  commonly used with testing modules used with SimpleTest where
  end-users should never enable the testing modules.