Perhaps they use some form of steganography.

Why would you expect one? I don't think I've heard of any AI image generators placing a watermark.

v2 has a watermark in the lower right corner of every image.

Thats the case now too if you use the API and not the labs version

why do you think this is?

There's probably no point in terms of a watermark's goals.

Additionally, since OpenAI is asserting no rights over the image, a watermark would be inappropriate.

Makes sense