If you haven't put anything on it and it appears consistently on other units of the same model, my suspicion would be that it is something on the rubber piece as built. Perhaps some silcone mold release is leaching from the surface (it is used on all molded parts).
Try to clean it off the rubber as much as possible (assume it's silicone) but don't damage the rubber. Then retreat the rubber with your favorite protectant, but remember a little goes a long way!!
Molding machine operators often get a bit carried away with the mold release spray, falling into the more is better trap. A part that tends to hang up in the mold often gets a bonus blast of spray in the troublesome area prior to molding. A simple wiping of the part after molding is usually all the cleaning it gets. The rest is left, evidently, to drip down your door panel....
Also look for sources above the rubber piece, ie, it may be coming from higher up and just travelling across the rubber part before you can see it. Metal window trim, etc can have rust that does this.
good luck.
