An optional hyphen can be seen if you have nonprinting characters displayed. It will appear as ¬. Without nonprinting characters displayed, you won't see it unless the break falls at the end of a line, in which case a hyphen will be displayed.
You should not be using a hyphen with inclusive numbers (such as dates); instead you should use an en dash (which by default is a breaking character). Like Stefan, I don't understand what trailing spaces have to do with it. If the issue is the last line of a paragraph, then either the line ends in a line break instead of a paragraph break or you have inadvertently used Distribute Text (Ctrl+Shift+J) instead of Justify (Ctrl+J).