If regular search does not work, try the built in scripting, of which you can use Javascript, Visual Basic Script, or AppleScript to code. I'm going with JS...
I'm no expert, but I found this code snippet from page 101 of InDesignCS5_ScriptingGuide_JS.pdf and modified it a bit:
var folder = new Folder("C:/Path/To/Files");
var files = folder.getFiles('*.indd');
for (var i=0; i<files.length; i++) {
var file = files[i];
open(file):
var myDocument = app.activeDocument;
//Clear the find/change text preferences.
app.findTextPreferences = NothingEnum.nothing;
app.changeTextPreferences = NothingEnum.nothing;
//Search the document for the string "Important Text".
app.findTextPreferences.findWhat = "Important Text";
//Set the find options.
app.findChangeTextOptions.caseSensitive = false;
app.findChangeTextOptions.includeFootnotes = true;
app.findChangeTextOptions.includeHiddenLayers = true;
app.findChangeTextOptions.includeLockedLayersForFind = true;
app.findChangeTextOptions.includeLockedStoriesForFind = true;
app.findChangeTextOptions.includeMasterPages = true;
app.findChangeTextOptions.wholeWord = false;
//Perform search
var myFoundItems = myDocument.findText();
if (myFoundItems.length) {
alert("FOUND!");
break;
}
app.findTextPreferences = NothingEnum.nothing;
app.changeTextPreferences = NothingEnum.nothing;
myDocument.close();
}
Don't quote me on that, I did not actually run the code, but that's the idea.