Ever the techy, I figure that just means we don't have enough sensor capability. All the Iraqi scuds (and several other of their missiles) were liquid fueled missiles that used red fuming nitric acid as the oxidizer. This stuff is always out gassing and the fumes should be detectable from quite a distance by viewing in the appropriate optical spectrum or using semiconductor NOx fume detectors. Follow the NOx plume back to its source and that's a gotcha. As long as we weren't tracking camel farts, this might work. Equipping a UAV with this technology could go a long way to locating scuds.
Regards,
Boot Hill