In the dialog for the "Wix Compiler (candle)" action, on the "Parse Options" tab there is a checkbox for "Show warning messages". When this is checked, FB sends in "-sw" as a command line parameter to candle.exe. However, when listing the candle.exe parameter list it states:
-sw suppress all warnings (same as -w3)
I am not sure whether its the candle parameter help that is wrong, or if FB has the checkbox reversed but clearly there is a mismatch.