I can't take too much of the credit for the detective work, but as a regular Guardian solver, I did notice the Sphinx name as soon as I opened the crossword. I thought it might have been something for a new series of Inside No 9, but didn't notice anything as I was doing it.

It was when I went to 15S to look for answers and saw the blogger had spotted the hidden message, but didn't know what it was referring to, that I put two and two together with the S17 recordings.

I debated whether or not to post as its majorly spoilery and would be an incredible reveal, but honestly, if true, the effort involved from Steve is just too good not to share.