Does using HVDC for distance transmission make theft of electricity easier to detect?
No, using DC rather than AC doesn't make power loss any easier to detect.

But it does make theft much harder to begin with. With an AC transmission line, most forms of theft rely on the AC magnetic and or electric fields radiated by the lines &mdash; i.e., there is no direct connection to the line at all.

With a DC line, there is no radiation of that type, other than any noise generated by the power converters at each end, and the random current variations caused by customers switching loads on and off.