So can you technically "hide" data in a ZIP file as a form of steganography? Just add it to the zip file but mark it as deleted. Then any common zip utility will skip it, but a specialized one could extract that extra information. This would be easy to miss as the size of a zip file is always different than the sum of the size of the files it contains.

And of course googling zip file steganography reveals this is not a new idea: https://aroralakshay2014.medium.com/hide-it-lest-find-it-zip...