When is node_search() in the node module called?
node_search() is the node module's implementation of hook_search(), which:


  allows a module to perform searches on content it defines (custom node types, users, or comments, for example) when a site search is performed.


That hook is invoked from a number of different places.

At a guess I would say if someone was going to make changes to node_search() it would probably be for the 'search' operation (i.e. altering the results of the standard node search). That particular operation is invoked from search_data(). 

It might be worth having a look a regular copy of node_search() alongside your hacked copy to see if you can understand what the differences are, and whether you can revert the changes without breaking things (if that's what you need to do).