That's because Off is not truly Off, its in a standby mode waiting for the touch of the button or a power signal from the dock. The standby circuitry they use just draws a little too much power. I like a nice switch that disconnects the battery when off. That way months later when you turn it on there is still a majority of the charge left. Doesn't help with dock signals but you can't have it all...
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Many thanks for the explanation - makes complete sense, I'm with you on the "totally off" switch - that would be great.