When Find or Replace is submitted by Enter while Ctrl is held down, perform the search from the beginning of the document. This could be done manually like this:

1. Put a caret at the beginning (Ctrl+Home)
2. Invoke Find or Replace
3. Submit it with Enter

However, unlike these manual steps Ctrl+Enter does not change the caret or scroll position if there is no match - they remain as they were before dialog submission.