Every time you leave Herschel make sure you use the Log Out page.
It's important to close Herschel properly so your temporary files are cleaned up and your data files aren't damaged.
When you log out, Herschel can delete any PDF files you've created. If it can't delete a file, perhaps because you have it open in your PDF viewer, then Herschel will warn you it hasn't deleted all your PDF files.
Note if you don't use Herschel for 90 minutes it will attempt to automatically log you out. This is to make sure that at the end of the working day, no Herschel users are logged in and your backup will be able to copy all the Herschel files. Note that Herschel won't automatically log you out if you are editing any data.