Modern nukes are not that easy to detect.
After about 20ft plutonium is hard to measure. Especially if it’s wrapped in a heavy metal.
Efforts have been underway since at least 1959 to detect nuclear weapons remotely, even while shielded, both by detecting high-energy particles directly emitted and by detecting changes to air molecules in the vicinity of the nuclear material (again, even while shielded). Breakthroughs occurred when advanced remote satellite sensors were launched in 1989 or shortly thereafter, and again more recently when it became possible to integrate the sensor data from numerous orbiting satellites along with ground-based and aircraft sensors. Among the classified technologies employed is the ability to remotely interrogate the composition of the nuclear material to identify the isotopes present and identify the origin.