The likelihood of spotting a transiting planet is a function of the orbital radius and planetary radius, if I recall. If it's big and close, it's more likely that you'll see it if for no other reason that it will modulate the light of the primary frequently. Farther out means a much bigger volume of space that can miss the planet. As it turns out,
http://www.google.com/search?hl=en&q...&aq=f&oq=&aqi=
has a lot to say on the subject.