I have just realised what's happening - it's my mistake!

The "Mark as...." and Junk Dustbin options are greyed out if I do a non-specific search, i.e. "What's New".

If I do a specific search, i.e. enter keywords or song titles, then the options ARE available.

However, I don't understand why you cannot mark something as junk in "What's New".

If you see a file that's supposed to be a well-known song by a well-known singer/band and it's only 15.3KB, then it's obviously junk and you should be able to mark it as such.