Yeah the UV light is a test for something else (think it shows UV sensitive inks). The pens and even lights and what have used at point of sale are known to be unreliable. But they’re reliable enough to spare a business the expense of counterfeit bills most of the time. And there is more than 1 type of pen out there.
There are much more reliable versions of chemical and light based tests to determine the authenticity of the paper and inks used. How do you suppose the banks make a determination? At the branch. When they process your deposit? Or law enforcement confirms counterfeits in the field? Then back at the office?
More over for the purposes of replacing destroyed currency you don’t need to establish 100% that currency is guaranteed authentic. You just need to catch the fakes often enough for it not to be a practical expense (like the cashiers). You can lose a grand here or there to plausible counterfeits. So long as its doesn’t happen often enough for you to become a regular target or impact the budget. My point was that there are very quick, easy ways for these people to do that, at minimal expense.
