Writing the desired phrase into the dialog box, then pressing Enter, results in no response or activity other than flashing cursor. Lib Off 126.96.36.199 in Ubuntu 16.04
If the searched phrase is not present in the document, the only response is the text “Search key not found” in the “placeholder for message” area. See the highlighted area of the attached screenshot. However, this message is somehow easy to be overlooked/missed. Please make sure that no such message has been displayed after hitting enter.