Tableau will not allow you to hide a sheet if it is not a part of at least one dashboard. Looking at your dashboard tabs, it may not be immediately clear where the sheet is, but it is there.
You may need to look at the list of sheets in the Dashboard Side Bar for each dashboard, one-by-one, until you find the sheet in question. If the Dashboard Sidebar is not showing, or if you're not certain what it looks like, click "Window" at the top of your Desktop Edition and see to it that "Show Side Bar" has a checkmark next to it.