I wonder if this has been found. It wasn't THAT hidden…

If you look around before you go up the lift to the final area on Cayde's mission (today's daily), there's vents you can break and climb through. Cayde then says it's his secret room, and you can take the glimmer but leave the exotics for him.

You get to the room, and find it empty. Your ghost says so, and Cayde talks about needing to get out of the tower. There's a card on the ground, which if you scan, will prompt a dialogue about Cayde's categorization system. Turns out hearts are for a girl he liked…

Other than that there was nothing. Am I missing something?

You're a little slow, as usual. ;)

Anyway, yeah I made a whole post speculating what that room could mean. I think it involves a bit of platforming and racing against time, because if you look at the placement of beams from when you first cross the bridge, it's almost entirely possible to make it up to that floor just by jumping. Heck, there's even a hidden walkway on the outside corner that leads into the final pre-lift room that you might never see if you don't look behind you after jumping a gap.