For at least the last 20 years, there are steganographic watermark systems in which inocuous parts of the images are used to hide coded data that stands up to even most lossy recompression.
Either HBO is using these and nobody knows yet; or they had an internal leak prior to their application; or they leaked it themselves. Under no plausible circumstances did they visibly watermark a corner and hope for the best.
I looked into stenography for a client who had a problem with audio being leaked by the users of his service. Even though there is a lot less signal in audio, it was trivial to encode a 32-bit pattern throughout the audio track in such a way that it was not discernible to his well-trained ears, and yet it would survive being encoded as a 64kbps MP3. We also checked pushing it through an analog loop, since that was most likely how his tracks were being pirated.
The pattern would repeat throughout the song (I think once per minute) such that only a small sample was necessary to recover the pattern. It also had enough check bits that it was impossible to get the wrong code; either you'd get back the right code or nothing. This was all done with COTS software (a command-line executable) that could be licensed for a few hundred dollars.
My client ultimately decided not to add stenography. He believed that his users would be upset with the possibility that there was some quality loss due to the process, as they were paying a lot of money to have high-definition tracks. He thought the losses from the discovery that he was applying stenography would exceed the ongoing loss from piracy.
> the losses from the discovery that he was applying stenography would exceed the ongoing loss from piracy.
A very likely correct conclusion, and there an additionally logical error with the assumption that privacy would disappear if they used it. Just because the copyright owner can sue whoever distributed the first copy doesn't mean piracy would stop after that. In order to get any effect at all, they would have to tell every customer about it, and the effect would likely not be 100%. It only take one person who upload the song to make it a permanent resident in torrent swarms.
If someone were to use this, the profit calculation is 1: Can winning lawsuits cover the costs, 2: Can informing (accusing) all your customers be worth the decreased rate of uploads. If neither of those 2 sound attractive to your business model, then spending money/reputation on personalized watermarks is completely pointless.
(edit) additionally, lets look at a typical scenario. Let say a file is being spread unlawfully and you track down it to customer 40#. The customer is a hardcore fan, studying for a music degree, and has no idea how the song "got out" when confronted. Customer 40# is against piracy, as can be seen by the major library of lawfully bought copies. However, they are also a member of a small fangroup, and did share the song with its 10-20 members. The customer was sure that everyone in that place would never think of uploading it to a public place. The question then becomes, do you continue with the lawsuit and put this student, this great fan in debt for the rest of their life?
That's certainly the right thinking for consumers. In my client's case it was B2B in the entertainment industry where licensing fees were per-day and substantial, so there are no poor students involved. Sorry for being vague as I don't want to link to him here without his permission.
I think he might have made the wrong decision in his case, as adding leak penalties to his contracts would have made his users more careful with his files. Since he was in turn licensing some rights from other parties, leaks in the past had caused him some legal trouble and he didn't want to have that happen again. His solution was to add DRM, but it didn't close the analog hole so there's still the potential for leaks. (I haven't talked to him in a few years so I'm not sure if things have improved.)
It seems that HBO gave copies of the four episodes screeners(?) to much more companies than previous years and this what's led to the leak. So since HBO is interested in leaking , i don't think it cares that much about protection.
And in any case - this won't hurt their subscription numbers - if you've wanted to watch the show, you've already subscribed to the first month and it's too late to cancel.
I wouldn't be surprised if they leaked it themselves so they'd have download numbers that they could use to evaluate the success of HBO Now (their over-the-top offering).
I suspect this is the case. It seems like people are assuming HBO's silence means they didn't use a steganographic watermark. I disagree; I don't see any reason why they would feel the need to tell the world - especially if they plan on pursuing this legally.
Either HBO is using these and nobody knows yet; or they had an internal leak prior to their application; or they leaked it themselves. Under no plausible circumstances did they visibly watermark a corner and hope for the best.