What makes alcohol-free beer sweet? Yesterday, I had a can of alcohol free beer which tasted almost like normal beer, just without the alcohol bite. Practically any other alcohol free beer I've had has a different taste. They all are slightly sweet (but not with a sucrose sweetness) and remind me very much of the taste of young corn, specifically the white liquid contained in the raw kernels before the corn is ripe. This happens with beer which contains enough hops and no corn at all (brewed by Reinheitsgebot), while alcoholic beer which actually contains corn (like Tuborg) doesn't have that taste. I am talking about regular light lager here, not the strongly malted dark beers which turn sweet with caramel products. 

I think that young corn is sweet because of its glucose, which converts to starch while ripening. Is it glucose which makes the alcohol free beer sweet? If yes, does it ferment in the alcoholic beer, or is it still there, but covered by the alcohol taste? Or is my sense of taste misleading me?
Beer is made from malted cereal grains. These can be corn but more commonly malted barley. The malting process turns the starch in the grain into sugars then the fermentation turns the sugars into beer. To get a strong dry beer, first start with a sweet malt and the ferment until as much sugar as possible becomes alcohol. For a non alcoholic beer, you need to remove the alcohol so you start with a low sugar malt and ferment just long enough to get the flavour but not too much alcohol. Since the fermentation is short, even through the initial sugar content might be low, there is sugar left in the beer thus giving you a sweet taste.