Originally Posted by
UA^PX
Nothing will happen in terms of "penalty". This is considered a NO SHOW. Every ticket has a no show policy which says: In event of no show, the remaining ticket will be considered forfeited. It's in most AC's ticket policy for Reward tickets.
Only one of the many rules that apply to a ticket is the "no show" policy. The other policy applicable here is hidden-city ticketing, much harder to prove and very unlikely to result in any consequences for anything less than repeated and obvious circumvention.