A user is seeing some hours against the Sick Time Used field in the Personal Time Off Requests screen, but isn't sure where that value is coming from. There aren't even any time entries logged against the pre-defined activity for sick time.
This is probably because you have changed the default activity item for tracking sick time in Global Settings > Master Information > Time Off. If you have set a different activity item for tracking sick time there, the hours logged against that activity show up in the Sick Time Used field in the PTO Requests screen. You should set the activity item to the one that you are actually using to track sick time, so that the value seen in this field makes sense. Check the Core Help for more on setting the default activities for personal time off.