The Radius Global/Radius Central scam takes advantage of an actual international market research firm, RADIUS, based in New York. The firm knows nothing about this "Secret Shopper" research scam...it is a nuisance, for sure. They have no secret shopper research program.
The scam usually involves a few emails, and suddenly a Priority Mail envelope with two odd-looking home-printed Cashier's Checks inside, with a form to fill out. This is the "Trial Assignment" with some fake deadline to encourage you to rush the process before the "Cashier's Check" clears your bank. It's tempting becasue you get the potential to be over-paid ($300-$400) for each research assignment. This is the "hook" of the grift and should serve as a red flag to you.
You get instructions to deposit these checks and quickly buy Money Orders at a Western Union or USPS location, ostensibly using your own money and fill out some sort of form with "research questions" on it. Then, you are instructed to forward the Money Orders and "research" to them and then find out the checks are fake, and you are beaten out of your money. Beware!