Metal Gear Forums

Master Collection Vol. 2 doesn't allow you to unlock specific weapons, items and music tracks in MGS4 by simply entering passwords in a menu after clearing the game once. You must take photos of special locations on New Game Plus to do so.

Here's a non-exhaustive list thanks to various X members. The photos must be taken on New Game Plus.

EDIT - Act I - The naked statue - Mosin Nagant
Act I - The cybernetic wing in the Rebels' compound - iPod track Mobs Alive
Act II - The trail of clothing items (blue scarf / pink bra) - iPod track Desperate Chase / Drebin FaceCamo
Act II - Crop Circle - Scanning Plug S
Act II - Eye test - Altair outfit
Act II - PMC soldier urinating - Type 17 Pistol
Act II - The statue depicting the human anatomy - Desert Eagle (Long Barrel)
Act II - A radio in a building - iPod Track Midnight Shadow
Act III - Statues in the park - iPod track Subsistence Action
Act IV - The "H" sign on the Heliport - Thor .45-70
Act IV - The destroyed camera on stairs - 1911 Custom
EDIT - Act IV - The broken electrical floor generator - Mk. 23 (SOCOM)
Act IV - The locker inside Otacon's lab where the latter hid before the Gray Fox fight - Patriot
Act V - The 3D worldmap - iPod track Gekko

EDIT -
Video with all the locations. I updated the list above according to it.