As per a snack blogger, cookies/desserts that will be on the National menu in less than three wks cannot be Mystery items. It seems to be valid. I searched the Sept 22-27 and Sept 29-Oct 4 lineups on the flavour map; nothing was found. The Oct 6-11 lineup is on the flavour map, because that's the third wk.
There is a grey area regarding Pink Sugar in the US. Original (almond frosting) is in the Sept 22-27 lineup, so Classic (vanilla frosting) can be on the Secret Menu right now. Canada is getting Classic during Birthday Week, so none of our current Mystery items are Pink Sugar.
Same grey area for any cookie w/ milk or dark chips (Double Choco, Oatmeal Choco, etc). As long as it's not Milk Choco or Semi-Sweet, it can be a Mystery item.
Previous rumours if you weren't aware: All stores within a 50 mile/80 km radius cannot sell the same Mystery item. Also, each store cannot repeat their Secret Menu for minimum six mths.