The signal from your Fitbit can travel through walls and ceilings, so your Fitbit could be directly above or below you on the next floor, or in the next room when the indicator says Warmer or Hot.
Also make sure to check locations where your Fitbit may be hiding, like in the pockets of clothes, under the bed or couch cushions, or under the seat of your car. Fitbits can fall off in weird places!