I raised the case with WD Support and got a response which included the following…
"As I understood, your cloud device does not go to sleep mode and you are afraid it may be hacked.
Your device is not hacked. This situation is perfectly normal for a cloud drive. Especially if the Cloud Access or any other remote access services are enabled.
Additionally, one of the main actions that makes the drive to stay awake is processing large quantities of media files as each picture, music file and video must be indexed in the background.
If you happen to be loading new files to the drive very often or changing files each day, then it is an expected behavior.
This is nothing to worry about, as cloud devices are designed to work 24/7. All you have to do is to make sure that the drive gets cool air and is dust free so it would not overheat.
If you would like your drive to be shut down for the night, you can use power schedule instead."