Unfortunately the ENL in flight recorders records noise. It does not record when the engine was running. The software has to determine whether or not an engine was running at the respective recorded noise level.
Our algorithm for detecting this is quite sophisticated, as it works with changed averages and at the same time compares the vario readings to them. But it can go wrong. For this reason there is an option to declare a glider as "pure glider" or to manually set when the soaring ended (or begun). You can setup all that through Edit->Flight Properties dialog.
If you are flying a glider without an engine the best way to protect from these unwanted problems is to turn ENL recording off in your logger.