Make sure you are walking very slowly and check the "Last Seen" time.
Bluetooth signals only come once every 5 seconds on Apple, and much slower on Android. If you are moving around too quickly, you might receive a hot signal and move to a different part of the house before it updates. If you get a hot signal in your living room, and keep walking into your kitchen, the indicator may still read hot for another minute, even if your Fitbit is in the living room. That's why it is so important to walk slowly.
When the app reads "Hot," make sure the "Last Seen" time is updating frequently. If the "Last Seen" time is not updating, your "Hot" reading is not current. Walk slowly to a different part of your house. If your "Last Seen" time is updating frequently, you are very close to your Fitbit.