Often when installing an app there's a time when you're asked if you want the app to launch at boot time. That adds the kext files in the system but it's not listed in the Login Items preference pane.
Launch the Box application again and see if there's any place you can turn off the launch at login feature.
If there is none you could uninstall Box according to the developer's instructions.
If all else fails download and run the shareware app Find Any File to search for any files with the application's or the developer's name in the file name. For the Box software I'm guessing you'd do the following search:
1 - Name contains box
Any files that are found can be dragged from the search results window to the Desktop or Trash bin in the Dock for deletion.
FAF can search areas that Spotlight can't like invisible folders, system folders and packages.
If you're unable to drag them to the Trash because they are running boot into Safe Mode according to How to use safe mode on your Mac and run Find Any File from there. Then rReboot normally.
NOTE: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following;
• Verifies your startup disk and attempts to repair directory issues, if needed
• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)
• Prevents Startup Items and Login Items from opening automatically
• Disables user-installed fonts
• Deletes font caches, kernel cache, and other system cache files