Offline
An iPhone or iPad 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 of your iPhone or iPad — AirPods are in range within about 33 feet (10 metres)
A Beats product is offline if:
it is turned off
it is out of range of your iPhone or iPad (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.