oh - good point airportworker - i should have said i was talking about the USA -- i can't speak about what's done in Europe or the UK. but here, travelers are required to provide TSA security info - passport number, full legal name & DOB, etc., for international airline tickets. - thus, any/all upcoming flights a traveler is scheduled on, will also show up on the TSA computers when the airport security ppl scan the traveler's passport.... so it will be seen that the passenger has booked a return flight. It will just have it's own separate itinerary and 'record locator', & may be on a different airline than the outbound flight --
but in any case, the TSA can see that the person isn't leaving the country without any return travel plans, so it won't necessarily raise any flags for TSA/security to question the passengers, or tag their luggage for special search - of course it might - but it's just a possibility.
for me personally, saving significant amounts of money, and/or - getting significantly better scheduling/routing by buying two one-way flights, is totally worth the small chance of being 'flagged' or questioned; I always have printed copies of all flights & itineraries on my person anyway.