You could use the Opus Find command, with appropriate arguments, to automatically launch a search which tells you which (if any) files contain the string you're interested in. Let me know if you want to do that and I'll work out the parameters. (Also, will you always search for the same string and in the same path?)
The drawback to this is that while it tells you the names of the files which contain the string, it doesn't show you the lines inside the files. Sometimes that doesn't matter but often you're interested in the context that the string appears in. For that kind of thing you might want to look at using the findstr command that comes with Windows. The Grep Dir button in my Diff/Merge Toolbar is an example of using this from Opus.
Shout if you need any assistance!