I am sure the image is in the tombstone directory.
Eg:
https://www.example.net/uploads/default/original/4X/1/3/f/13f5a8a1b636146333dacccecf1a98dda9facc71.jpeg
404 Error.
https://cdn.example.net/uploads/default/original/4X/3/4/6/3467394a5f188e390e108a0275335d08bd949d2f.png
200 //have CDN cache Hit.
https://www.example.net/uploads/tombstone/default/original/4X/1/3/f/13f5a8a1b636146333dacccecf1a98dda9facc71.jpeg
200 // in tombstone directory.