Brewbirds, it is possible to develop a leak at any time. Howver, what is most likely is for the leak to occur at the time of surgery or during early weeks of initial healing. It may take years to develop into something diagnosable, because the leak is so tiny. But, in reality, it was always there. It was just not detectable.
If you have unexplained symptoms that defy all imaging and other diagnostics, sometimes the culprit is an indolent leak/sinus tract that is only fully discovered by surgical exploration. We'd like to think that CT and MRI technology gives us perfect views inside the body, but they remain limited. You only wind up seeing what the contrast can get to (either by cavity flow or bloodstream). The rest is a black void that appears fine. It is only when things change to affect adjacent structures that it becomes more apparent.
Also, the skill and experience of the practitioner viewing the films can make a huge difference. The doctor that keeps looking despite negative results is the true puzzle solver.
Jan