If you're running Windows, (or perhaps your files are on a network which includes one or more Windows machines), find and install a search utility named Agent Ransack. It's the free "light" version of a more sophisticated product, but it will find the linked file's name referenced in the "code" of InDesign files.
For example, a search for *.indd
...with linkedfile.ai entered in the "Containing Text" field
...will hit all the InDesign files in which linkedfile.ai has been Placed.
It's awesome and works every time.
