The way I read the info posted by Mike a few weeks back, is that the watermark does not exist within the stream, but is inserted by the player during the process of decryption.
In AACS 2.x the “decryption” process (by a player) is rather different from AACS 1.0 - compliant device not only decrypts the content but also actively meddles with it. Forensic mark is always inserted into video stream at the decryption layer. This mark contains player model and serial number along with environmental details. This information is preserved in video stream and survives screen / HDMI capture.
Since this happens on every playback, there’s no reason for the player to call home to report this into (you’d just get a million players calling home every day to report someone watched a disc). The true benefit is from being able to track those who capture and share the vids on torrent sites using the details contained within this mark. Whether or not this will have a meaningful impact on piracy remains to be seen, but it’s not hard to see how it may at least give a few people pause.