What terms did your google search use? A quick google for "listerine ingredients" brought up a label [0] that shows caramel (sugar) as an inactive ingredient. Only one I could find though. Most others however do seem to use artificial sweeteners
"Caramel" in this case likely refers to caramel color. While it is still made from sugar, I doubt there's a significant enough amount to consider it "added sugar". If you look at the other colors of Listerine, there is no caramel listed.