Metal detectors already know if you're concealing a weapon, and eventually they might know if you're concealing large sums of cash too. University of Washington physicists Christopher Fuller and Antao Chen have discovered that American banknotes contain enough magnetic inks that metal detectors can actually pick up their signal. For instance, Fuller and Chen found that a handheld detector could pick out a $1 bill from three centimeters away — which may not sound impressive, but with every five additional bills the strength of the signal increases. It's possible to know how many bills a person is carrying, but because all the bills contain the same amount of ink (i.e. a $5 bill is the same as $100), the machine won't know how much each one is actually worth.
With more powerful detectors, like those found in airports, it might eventually be possible to find people smuggling cash simply by scanning them. But the technology's not quite there yet, as the excessive magnetic noise found in real-world situations can make separating bills from other magnetic material difficult. "A lot of development work is needed to make it practical," admits Chen.