Is difficulty not just synonymous with cost or importance? If looking through the window is not an option, they can either to the XKCD route (spanner or hammer) or do the "phone up your phone company records" route.
You would need to have something very important for them to bother that much. But a couple of videos on Defcon shows it's not as hard as most people think it is. Some pretty good demonstrations on there.
PS, I'd also love to see what could be done with the maths involved in detecting signals below the noise level. Some really clever "tricks" can be done with the right kit or right algorithms.
Biting the hand that feeds IT
