How would I design a cable line that doesn't leak heat to the ground surface?
There are several ways to make cables undetectable in terms of waste heat:


burring them deeper
increasing conductor diameter
increasing the voltage


In the end, all these solutions cost extra money and are not implemented in practice, AFAIK.

Furthermore, there are more reliable detection techniques than observing snow patterns, such as metal detectors.

If fact, I would question your statement about underground power cables being secret. Information about such cables is actually widely available. Have you ever seen this sign?