I'm pretty sure there isn't a scale detector in the machine and the "de scale" prompt comes up after a certain number of pump runs over seven seconds, or total seconds of pump run or something like that.
I'll have to start with the (dis)qualifier that I have never de scaled my -920 and I don't ever plan to. Not only because of the stories of de scaling's gone wrong, but because I use scale free water like LaMarzocco recommends or like the shops with their commercial machines, so take with a grain of salt...
Something I read on the Aussie forum said this machine likes "threes". What you might try is unplug, tap the power switch. Plug back in. Do factory reset by holding the 1 cup button while pressing the power button. "Reset" will appear in the window. Press the "menu" button and the machine will factory reset and ask for your water hardness (which I set to lowest). Do the whole factory reset as described, three times ("Breville rule of threes ").
I do three consecutive factory resets about once a month in order to avoid getting the "de scale" prompt.
And hey, report back how it goes, would you? I'm curious.