There is a possible clue in my case, although I doubt that it's relevant. In IE I had looked up a type of car, and in the web page there was a link to a YouTube clip of it, so I opened it. Afterwards I closed by using the back button and tapping the X. Only some time after the next morning's battery drain did I happen to scroll down on the Start screen and notice a new tile at the bottom....I must somehow have unknowingly pinned it. I made a screenshot of it and here's the relevant part.
Of course I unpinned it afterwards....I don't know what happened to it, it's nowhere among the list of app tiles. I already have a proper YouTube tile.
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