I agree that Mp3tag should probably keep any selected files after you've cleared a filter, and it should keep the scroll location (to the first of the selected files, I suppose, because you can't scroll to all of them if they're not all in view within the window). That would seem (to me) to be a relatively small enhancement.
But I'm not sure I understand how a Find function might work.
Which fields would be searched? All of them?
And what would happen in the user interface? Would all of the files that are "found" be selected? Or would it take you to and select only the first one?
Personally, I'd have little use for such functionality, but I know that other people use Mp3tag differently. I've always said that there are very few instances where the typical user needs to be working within their entire music collection all at once.