I have 2 profiles, offline and online. I only use my online profile for downloading DLC, it has zero achievement. So if I play a game before it released on store, strictly on my offline profile, next time when I go online with my other profile, will microsoft scan all the game I have played including the one i played on my offline profile?

I would assume they can see everything on your hard drive. My "offline" profiles used for testing and such are stored on a usb stick, or immediately erased when done with the game, prior to going back onto xbox live. Especially with unreleased games or anything modded, ie, the leaked mortal kombat complete edition!