Isn't the first rule of fraud detection not to talk about rules you use to detect fraud? ;)
Merchant-side fraud prevention schemes are not security. They are heuristics for reducing the number of bad transactions that the vendor has to handle.
Algorithms should be air-tight. Yes, the best way to ensure that they are is to make them public. Heuristics by definition are not airtight. The best way to get utility from them is to keep their nature hidden from parties trying to abuse the system.