The ability to give detectors "targets" (meaning, they can only detect those specific units) would vastly expand the stealth function. In fact, to be honest, Military will eventually need this function, as there's more one type of "stealth". For example, the Stealth Bomber is stealth, but can only be detected by the AA Launcher. For now, that's the only stealth unit, so that's fine with the current system, but eventually, there will be an opposing faction and spy units, which will use a different stealth system, only detectable by each other. Thus, the need for detectors to specify what units they can detect.
This is best done by making the detector tag something like this:
<detector value="true">
<unit value="stealth_bomber" />
</detector>