I was also completely blown away by something I only discovered years after playing the game when the sequel landed.
There's a portion in the game when your brother Paul is sick and he tells you to leave him to fight off enemy goons alone. Apparently you can fight your way through the hotel and he survives all the way to the end of the game!
When the sequel came out, I was like "why the fuck is Paul still alive"
Indeed he does. There are many of these hidden things.
You can kill Anna on the plane where you're supposed to kill Lebedev; the game accounts for this with voice dialogue and everything. Crazy. At that point in the game there is no reason for you to even try (and she's a tough match) unlike later on --- but you can!
Haha, nice. Which reminds me of my first playthrough as well, and I was very much on-rails brained (just do what the game tells you to do, because most games give you barely any agency). In the plane where Anna goes to kill Lebedev, for whatever reason I tried killing her, it took a few tries but I ended up LAM'ing her and I was confused that the game continued afterward.
The game implements this in a funny, low-tech way: you enter through Paul's window, and if you exit back through his window, the game treats it as though you've chosen to abandon him, so he dies. In order for him to survive, all you have to do is exit through the front door of the hotel before he is killed by any enemies, which you can do by running as fast as you can past all the enemies.