Very helpful 2mph!
My car is doing that, too (could be a photo of mine). Very subtle...literally just a few drops now and then in that exact same spot. I did replace the cap...and it still does it. Since the overflow tube is non-pressurized I'm thinking that RSO 34 has the fix. It must be coming from a minor leak near that top fitting on the bottle itself. Doesn't seem to be life threatening, as only a truly trivial amount of coolant is being lost (at least given the frequency I drive mine). I plan to replace it sometime over the Winter. Please give us an update if you get to it sooner...