Per 2017 NEC all outlets in the garage and outdoors are not required to be AFCI protected, except a laundry area in the garage requires AFCI protection. Since the FAU is not mentioned, I think if in the garage or in a utility enclosure that opens into the garage or to the exterior, it is not required to have AFCI protection. The circuits running to these areas do not need to be in metallic conduit either. Up for debate is a FAU enclosure which opens into the dwelling unit. I think this is not a closet or storeroom, so still not required, but some inspectors may disagree. I do not know why people are talking about RMC...the requirement for fire alarms, home security, and snow and ice melting is metallic conduit so flex, EMT, AC, and MC will comply. Of course any connection to a FAU is an outlet...an outlet which I believe in most locations, does not require AFCI.