Exact current information on "missing a return flight"?
How, specifically, do airlines distinguish between people who miss a return international flight for genuine reasons, and people who skip specifically the return leg of a ticket to get a cheap one-way flight on an international flight?
What is the actual legal and/or contract basis of this? Does anyone have any factual experience of it?
For example: when this happens (you genuinely miss or can't take the return leg) does (for example) someone from the airline try to telephone you? Letter, email? Are there any factually known procedures or happenings?
(As opposed to the oft-repeated general idea that the airlines will "go after you" or "try to charge you" in such a case - which is possibly just a myth; hence this question.)
The example fare I had in mind was Europe to the US. In broad general terms this is about 700 euros for round trip but almost always about 1700 for a one-way, so it's a big difference - thousands and thousands of euros for a large family, say - you can imagine the airlines being pissed. {BTW if you're looking for such a fare, a rare exception presently (2015) is XL which offers cheap one-way flights.}
Best Answer
Based on your premise that the family does call in the fact that they need to cancel their booking due to a medical emergency then they have duly notified the airline and their missing the return flight would not mark them as "skipper-outers".
Most of the major airlines do not sell "non-changeable" international tickets. The cheapest fares are "non-refundable", but almost always changeable for a fee. Some of the cheapest LCCs do sell "non-refundable & non-changeable", but they aren't part of your scenario. So in your scenario the family will likely have been offered the opportunity to reschedule and hopefully their insurance will cover the change fees due to the medical emergency.
If they skipped the flight without notifying the airline, then the gate agents would flag their booking as a no show. And as they are a family of five from outside the country, then it maybe that someone else in the corporate office will review the booking and determine if any follow ups are warranted.
The airlines do pursue "skipper-outers", but not all. Mostly it is a issue of cost to be recovered vs legal investment to do so, serial "skipper-outer" vs one time infrequent flyer.
Pictures about "Exact current information on "missing a return flight"?"
Flight From WW2 Era Returns After Years Of Missing
More answers regarding exact current information on "missing a return flight"?
Answer 2
To answer the concrete question, how airlines determine who is being malicious and who is actually missing their flight?
By frequency.
The airlines have IT systems that allow them to statistically track all fliers. They know what happens in 99% of the cases and how frequent cancellations or non-attendance for the second leg are. When someone maliciously, once in a lifetime, drops the second leg, they won't know. It can be a coincidence. However, by the nature of the beast, people who are doing this are not once-in-a-lifetime fliers. They travel rather frequently and will most likely do this several times. On top of that, the people who do this only once are not as much as concern for the airlines, so they focus on people who do that frequently.
In their data, they can identify pretty quickly who is doing this regularly and then are able to start asking questions. But as stated in other answers, the possibilities for the airlines to do anything about it are very limited. Their best chance is to make it harder for you to find out the price difference and to not make it that clear to you that there will be no "real" consequences.
This is shown quite clearly in the fact that United Airlines tried to Sue skiplagged.com (which helps you booking those flights) end of 2014 instead of putting the pressure on customers using the service - because they cannot.
Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
Images: Madison Inouye, Prateek Katyal, Binti Malu, Anna Tarazevich