Frankly, if it can be detected, it can be removed. Makes sense no?
I imagine this is also the case for audio watermarks, but I'm not sure what the current state of the art is for watermark removal.
This is primarily because those watermarks damage the image. The information about the content of the image beneath the watermark is missing (replaced with the watermark) and would have to be extrapolated or fabricated to replace it.
If the watermark isn't visible as part of the image then there is no missing piece to have to replace.