I've updated the wiki with as much information as I can gather without possessing the Lenses themselves. In the meantime, here's a list of possible locations for other hidden chests in all the other Vecna-related content, rated by how likely they are to contain something.
The Icemount Curse and Immortality Lessons (extremely unlikely)
Vecna did technically show up in these two - although he was masquerading as a Mysterious Stranger at the time - but there was no sign of the Hidden Hand anywhere.
Seizing the Dawn / Beautiful Nightmares (very unlikely)
The Hidden Hand show up in both of these, but they're on someone else's patch, with no established territory - in fact, by the time you get to Beautiful Nightmares, they're all dead.
Eye Know Whodunnit / An Element of Chaos (unlikely)
These are the two quests in Peril of the Planar Eyes with any significant Hidden Hand presence - a little hidden cloister in Eye Know and a secret cave in Element of Chaos - but neither of them seem big or permanent enough for the Hidden Hand to bother hiding things there.
Dread Sea Scrolls (extremely unlikely)
No Hidden Hand, not even a direct appearance from Vecna...
Captive of the Hidden God / Fall of the Forbidden Temple (unlikely)
These are the only two locations in Isle of Dread I think are at all likely to have a hidden area. Captive of the Hidden God is the only quest in the chain to have the Hidden Hand at all (and that's one guy), and Fall of the Forbidden Temple is frankly just gut feeling on my part.
The rest of Isle of Dread (very unlikely)
Lots of Vecna, but no Hidden Hand - and they're the ones who've been hiding these things.
Order in the Court! (fairly likely)
There aren't many quests in the game where you walk into the Hidden Hand's turf, instead of them attacking you or you racing them to a Macguffin. This is one of them, though. We know Barington Worthridge is already in the business of hiding incriminating evidence for questionable acquaintances - why not a secret chest?
The rest of Grip of the Hidden Hand (unlikely)
There's already one chest in this pack, which is a good sign, but two quests (Army of Eternal Night and Three Paths to Battle) don't have the Hidden Hand at all, and they're only in a couple of bushes in Growing Pains.
Grand Theft Aureon, Law and Order, Party 101, Turn the Page, The Devils to Pay, The Evil We Know, What Dreams May Come (very unlikely)
I'm only really including these because they're in Vecna Unleashed - they've got nothing to do with Vecna or the Hidden Hand.
Paper Trail, Enemy of My Enemy, Vecna Unleashed, The Bark and the Blade
These four quests all have the Hidden Hand attacking places, rather than you attacking the Hidden Hand - that said, I can think of possible locations in all four. (The weird secret door near Haska Torlath's room in Paper Trail, Giv Chromar's portal in Enemy, Sir Jarus ir'Omik's room in Vecna Unleashed, and the Cannith ruin in Bark and the Blade, for the record.)
Taken in Hand (likely)
It would make a lot of sense for the Hidden Hand's largest base to contain some sort of hidden secret.
Fire Over Morgrave (almost certain)
There's got to be something here, right?