Photoshop sat hogging memory while doing NOTHING

I can't reproduce this on my machine. So I think it isn't a general bug. Unfortunately you haven't given us any information about your system. Therefore, all proposed solutions are a look into the crystal ball.

Is your system up-to-date? Do you have installed all recent updates and patches for the operating system and the device drivers, especially the graphic driver. If you have a NVidia graphic card installed, make sure that you are using the recent Studio version of the driver, not the Game Ready version.

Please provide a copy of your 'System Info'. This can be get from the Photoshop menu item Help > System Info. There's a copy button in the System Info dialog. Press this button and paste the info into your next forum post.

Please post ALL information from first line down to and including Plug-in Info. All other infos after the Plug-in lines can be cut out. 

Have you had files open? Memory is not released until you close Photoshop. This is by design.

Is memory usage steadily increasing with no computer activity?

Show screenshots from Task Manager > Performance - the memory tab and the GPU tab.

Also the full Help > System Info.

Hey, @JasonBatley. Welcome to the Photoshop Community. I'll need more info to help you figure this out. Please share the system info from Photoshop Help > System info > Copy and paste into a text document > Upload and attach here.