All's well that end well. Authorities recovered the stolen radioactive materials but we may not be so lucky next time around. How easy or difficult is it to detect radioactive material stored in a shielded container from the air?
It depends on a number of factors, so it is hard to determine.
One of the first is the type of radioactive material, which will determine what type of radiation is emitted, as well as the rate of emission.
The next would be the quantity of material (which will also play a part in rate of emission).
Then there is the type, quality and quantity of shielding. The type of shield to be effective is also going to be determined by the material type, since alpha radiation can be stopped by generally thin material since it is essentially the nucleus of a Helium atom and comparatively large. Gamma radiation OTOH is basically a highly energetic photon which can require several inches or more of lead to effectively shield.
Then of course there is where the radiation sensor is located relative to material, and how effective the sensor is generally, and at detecting the type of radiation emitted.
One of the old geiger counters I have played with was an old, yellow CDF geiger counter from the 50's or 60's, which no matter what I tested, never detected any abnormal radiation. I latter found out that that model was so insensitive, that it would take a lethal dose of radiation for the geiger counter to pick it up. Newer devices are of course more capable, though sensors mounted in aircraft could potentially run into issues depending on materials in the aircraft, and just the distance (and prevailing winds) from the source of the radiation to the aircraft.
-Cheers