To detect a plutonium-based weapon, my understanding is that even with the best detection technology one must be well within 100 meters for a discernible hit.
It depends on the size of the detector array. For an inconspicuous vehicle-mounted deal you’re probably right. If you could erect a gamma detector say 100 sq meters you could go out farther. If you had one 1,000 sq meters or so you could mount it to Art Bell’s triangle ship and hover overhead.
Nope,on rivetjoint flights they light up like the 4th of July