I think they do, and it's even scarier than SynthID. Look closely at the generated images and you'll start to notice some diamond-shaped high-frequency patterns in different parts of the image (tree crowns, vegetation, hair, textures, clouds). May be just a random bug, but for me it looks like the watermark is baked into the image details themselves and all the details follow this pattern. But it leads to unnatural results somewhere, i.e.: