Ragnar makes a good point; its more about awareness than specific cases.

OTOH, my DAP (Iriver) says 'Complete!' (with a huge full battery symbol) when its completely refilled, but the LCD is off. Only when I press a button or unplug the cable it says this. So when I look at it the next time, and attempt to use it, it says 'Complete!'. The difference here is that the LCD is off, but goes on.

The issue is that the device doesn't know a better way to contact the user because it isn't aware where or when the user is in the neighborhood and takes note of the device.

And ofcourse billboards cost electricity. Thats advertising. Advertising also makes corporations go round 'n about.