Or do you have something else watching the power draw and detecting the drop?
This. I have never done it myself, but I would imagine this would be fairly trivial with home assistant. The plug reports it's power draw back to HA regularly, and HA has a trigger for when [a value] goes above/below [a certain figure].
So set something up like "Plug currently consumption falls below 5W, trigger notification."
God I wish I lived wherever you did. That's a £300 machine, here.