Attack from any direction
If an attack can come from any direction and target any location then identifying radiological threat should be spread like a net over the whole area, not just around what are perceived to be the likely targets.
Too many potential targets
If the target for an attack could be anything or anyone, then a comprehensive always-on networked solution protects all citizens but allows for reinforced detection for apparent targets.
Avoiding radiation detectors
If the perpetrators of an attack or smugglers can see or can find out where radiation detectors are, then they can be avoided. Large volume, static detectors have a role to play, but this must be supplemented with radiation detectors that can be used covertly.
The number of detectors is one part of a complete solution
It might seem to make sense to flood an area with radiation detectors, but there is a problem with this approach. Protecting any location with a large number of non-networked radiation sensors (from large volume static detectors to portable radiation detectors) provides some protection for an area. However, it is reliant on the individual seeing the alert and taking appropriate action which given the number of things that could be going on at the same time may or may not happen. Even if the warning is seen and identified as not a false alarm, it can be challenging to track where the suspect package has moved.