You can have a PowerPoint process without a window.
One reason is you have preview pane enabled and have selected a PowerPoint File. To preview it an invisible copy of PowerPoint is opened and may not close when you deselect. This doesn't normally use much cpu though.
If you have any addins installed two things (at least) can happen. A badly written addin can have a close procedure that doesn't work. It may keep running when the visible window has closed.
Some AddIns deliberately open an invisible copy of a template as a 'library' if tyhere's a problem this file may stay open.
If you have AddIns try disabling them to see if it helps. If you have preview pane on try turning it off.
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