Even an audio analysis tool won't reveal much if the watermark is sophisticated enough. I believe, a basic watermark adds a few spots on the spectrogram in a kn

Even an audio analysis tool won't reveal much if the watermark is sophisticated enough. I believe, a basic watermark adds a few spots on the spectrogram in a known arrangement, a bit like a constellation in the skies, but a watermark can be based on the bloom filter idea: hundreds of barely visible spots arranged with a hash function will look like noise.