No OG, sugar isn't sugar. There is a whole suite of carbohydrates called "sugars", sucrose (what most people think of as sugar) is C12 H22 O11, glucose is C6 H12 O6 etc. Perhaps someone has found another naturally occurring sugar which is sweeter than sucrose.
Natural plant sweetener. Has an unpleasant aftertaste but doesn't break the bank to try it if you're interested. Been around in health shops for ages. Hit the supermarkets a year or twoish ago.
I can tell if there is sweetener in anything. I may not notice at the time but get a terrible aftertaste and sometimes a headache. I have to backtrack to see what I have eaten or drunk in the preceding 3 ish hours. Its quite annoying that manufacturers add sweetener to things without clearly announcing it on the label apart from in the ingredients list.