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Because it's just sleeping, not off, so it's still on your network. It works the other way too. I can see movies stored on iPad via my Mac's TV app even though the iPad is sleeping.
 
I thought it went to sleep when the lid was closed and needed an external monitor, mouse and keyboard to enable clamshell mode. And external power.
 
Thanks for that. I didn’t expect it to happen but it does!
 
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