Deleting Files that are in Use

Anyone shed any light on this

I have a Project consisting of a timeline with 20 'Chapters' . Each chapter, comprises multiple clips so I have made a Compound Clip of each Chapter so I can add the same effects etc / sound balance etc to each Chapter.

Media is External, I did not import in to the library.

Now I can Double Click on a Compound clip and see the individual Clips it comprises of in the timeline.

Now for any individual clip int eh compound clip, I can locate it in the browser, and I can move to Trash with Cmd Delete, Although it is in Use in the Timeline, it can still be deleted with no warning.

the Clip stays in the Timeline and if I right Click and 'Reveal IN Browser' the clip re appears in the Event Browser library.

I am trying to remove XS media to thin down my HDD usage , this is how I came across this.

It seems that you CAN delete "In Use" individual clips, , even though FCPX marks them as Unused, yet you can not delete Unused Compound Clips. I have a whole selection of the, not in use yet FCPX insists they are in use and won't let me delete them

Will these unused un deletable compound clips be taking up HDD space? Render files only maybe ? SO a Delete Render Fiels may help to reduce some HDD loading ?