Not sure, I am very sceptical that an X-Ray machine will always detect motors. Its very lazy policing by the UCI. The only way to stop any possibility of motor doping is to physically inspect bikes, including disassemble bottom brackets. You would need just two UCI mechanics at races to physically inspect the stage winner's bikes. All teams use Shimano, Campag or SRAM so the investment in tooling to quickly disassemble bottom brackets is minimal - much cheaper than X-ray machines designed to scan a moving peloton.
And if they actually do this its a powerful deterrent effect.
Here's another article. Lappartient seems more interested in PR than just taking simple actions - compulsory bike inspections for stage and race winners. If this was actually enforced nobody will dare try motor doping.
President David Lappartient also says that he is "not afraid to to catch a top rider" should they be using motored assistance inside their bike
www.cyclingweekly.com