Advertised today is a 241 deal to China with trip-a-deal and in the fine print is the following...
On purchasing this Travel Offer you are bound by the General Terms and Conditions, in addition to the specific terms and conditions outlined in this Important Information.
Full Service Airline(s) Used
China Southern or China Eastern Airlines plus any codeshare partners (subject to availability).
Please note: One way flights can not be accommodated. Travellers must take all included flights within the package, and any flights intentionally forfeited will result in later flights being subject to cancellation.
I wonder by what method they could determine
without doubt "intentionally forfeited"