When a SWMM engine run finishes it produces a summary report which is written to Notepad (as opposed to the Calculation Summary report when the Implicit engine solves). This report is a Notepad file and is not saved with the model once the file is closed. This report provides useful overall information on the run. If you want to review this result later in the session, you can minimize it instead of closing it. If you want to save it for later use, execute a File > Save command.