The universal search in studio is only targeted to search the opened XAML files. If I know where exactly the search item is in XAML file I wouldn’t need to use the search feature at all.
I think the search should target non-open xaml files which will make it easier to debug/fix errors.
Not sure if this isn’t working in earlier releases… but as long as they are within the same project, Universal Search comes up with any result, opened or not: