I’m not sure your comment about Activision-Blizzard legally being obligated to announce any sales is necessarily true. This may in fact be up to the new owner’s discretion, not Activision-Blizzard. Case and point: Codemasters (not Activision) announced that it would be publishing the new Leisure Suit Larry game. To give you another example, when Ubisoft picked up the Might and Magic franchise from 3DO a few years ago, Ubisoft made the announcement (and at its convenience), not 3DO.
Why I think Sierra’s other IP may have been sold: before (and shortly after) the big merger, http://www.questforglory.com and http://www.gabrielknight.com both redirected to http://www.sierra.com (and eventually http://www.activision.com). However if you type-in these addresses now, neither website redirects to anything anymore.