Hi Texbow,
Thanks for your question. Underground pipe detection is one that I am not familiar with but, if I had to make an educated response it would be the following.
1) You would want the highest possible environmental delta to locate potential anomalies. This being that the soil is significantly cooler than the pipes themselves. Though I am not the expert at this, I would not consider every hot/cold spot I detect as a leak.
2) The only way I could imagine to truly identify any major leaks would be include recording the inspection at least twice a week for a period of time.
3) One would want the highest resolution possible for detection. A 336 resolution would not be an ideal sensor for this type of job.
4) The size and depth of the pipes, and the type of soil it sits on would certainly affect your results. See #2
I've attach a video of a pipes line inspection, and incidentally came upon an underground anomaly from a pipe below the ground.