At 15:01hrs Truck 21 and Battalion 21 were sent on the MCFRS box for the reported residential structure fire. Upon arrival 1st arriving companies advised of nothing showing from a 2-story SFD however modified this to heavy smoke to the floor upon entry. Upon arrival Truck 21 set up on Side A and was assigned to assist Fire Attack Group with locating the fire while Battalion 21 was assigned as the Incident Safety Officer. While completing the 360 Truck 21A identified a hot area with his TIC and, upon entry, found a previously undiscovered bedroom which had burned extensively. Charging the line what fire remained was quickly knocked down with Truck 21 opening up for extension. Following completion of assignments all units returned to service.