FYI, I just noticed that on my Kindle Touch this watermark is stored in the SQLITE Collections Database at /var/local/cc.db in column p_watermark
in table entries
not just for DRMed books (p_isDRMProtected
=1) but for one purchased book where I removed the DRM as well (p_isDRMProtected
When I unpack said book with MobiUnpack the same watermark shows up in the BookName/mobi7/BookName.opf as
<meta name="Watermark" content="[...]" />
and is reported as an EXTH header when unpacking.
The cc.db also lists such a watermark for one mobi that I let Readability.com email to my Kindle. Strange ...