What is the ROOM icon for?

A couple of nights ago I noticed an icon on the notification bar which I had never seen before. I had no apps running in foreground and only the regular ones in background. When I picked up the phone the next day the icon was gone.

I looked up the icon from a chart of all Android Lollipop 5 icons. The name of the Icon is "ROOM". What is it for and why was it there at night and gone in the morning?

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