When you fight Psycho Mantis, he’ll say he’s reading your mind, but what he’s really reading is your memory card. If you have a save from Policenauts, Castlevania: Symphony of the Night, or Suikoden, you’ll get a special message.
Pyscho Mantis will also make a special comment if you’ve saved your game multiple times, or too few times.
After you’re tortured and thrown in the cell, there’s a way to make the Ninja come and rescue you. After the guard goes to the bathroom and Otacon brings you supplies, simply wait. The guard will go to the bathroom a few more times, and on the second trip, the Ninja will come and rescue you.
If you use your camera to take a picture of the orange robots in Otacon’s lab, the ghost of Hideo Kojima, the series creator, will appear in the picture.
Here’s a smelly but useful trick with the cardboard box. When you’re in the cave area with Meryl, if you hit her, she’ll order her wolf to attack you. However, if you equip the cardboard box before you get attacked, the wolf will simply pee on the box. From then on, whenever you wear the box, wolves won’t attack you.