Offline
An iOS or iPadOS device is offline if:
it is turned off
it is not connected to a data network, such as 3G, 4G, LTE or Wi-Fi
A Mac is offline if:
it is turned off
it is not connected to the internet using Wi-Fi or Ethernet
An Apple Watch is offline if:
it is turned off
it is not connected to a Wi-Fi network
it is not connected to iPhone
Note: Apple Watch Series 3 or later (GPS + Cellular) is offline if it is turned off or does not have mobile network service.
AirPods are offline if:
they are out of battery life
they are in their case
they are out of range from one of your iOS or iPadOS devices (AirPods are in range within about 10 metres)
A Beats product is offline if:
it is turned off
it is out of range from one of your iOS or iPadOS devices (the range depends on the model of Beats product)
If a device is offline (or if Find My is trying to connect to the device), a grey dot 
appears beside the device in the My Devices list.