Most surprising discovery/secret/unlock/experience in this game?

[NateDog]

This might seem like an odd pick but for me when I first played it was the Sneaking Suit.

I was used to finding smaller items in lockers but it felt mad to me that Naked Snake's Sneaking Suit, which was the default in MGS1 and 2 narratively, was a missable item hidden in a locker in Groznyj Grad. Felt like a great reward for exploring the location when you're meant to be sneaking through it, and it being similar to The Boss' just in time for the final stretch leading up to her fight made it that much cooler to me at the time.

I was just coming here to post this. Because I'm an idiot I had never actually found it in-game until I replayed it a couple of years ago on Vita and randomly thought "I wonder if anything is available in that locker that Raikov was in". That being said MGS3 was and still is by far my least replayed of the numbered MGS titles ignoring V so I suppose I only had 3 chances or so to find it before. Still ridiculously stupid of me though given I had looked at stuff like the Hot Toys figure over and over again and never said to myself "why didn't I have that suit in the game?".

I'll also go with giving poisonous food to The Fear, always gets a laugh out of me.