There are two bonus items per scenario. Get an "A" or better rank under the Normal difficulty setting to unlock the first item. Get an "S" rank on the hard difficulty setting to unlock the second item (and first item if not already unlocked).

Item	How to unlock
Battery	Dark Legacy 2 with an "S" rank.
Blue/Green Leech Charm	Train Derailment 3 with an "A" rank.
Blue/Red/Green Chemical	Umbrella's End 2 with an "A" rank.
Blue/Red/Yellow Gemstone	Rebirth 2 with an "A" rank.
Book of Wisdom/Future Compass	Raccoon's Destruction 1 with an "S" rank.
Briefcase	Train Derailment 1 with an "S" rank.
Chronos Key	Raccoon's Destruction 3 with an "S" rank.
Clark/Gail X-Ray	Rebirth 1 with an "A" rank.
Crystal/Obsidian/Amber Ball	Raccoon's Destruction 3 with an "A" rank.
Cylinder Shaft	Nightmare 1 with an "A" rank.
Death Mask	Rebirth 2 with an "S" rank.
Eagle/Serpent/Jaguar Stone	Fourth Survivor with an "S" rank.
Emblem/Gold Emblem	Nightmare 2 with an "S" rank.
Film A/B/C/D	Umbrella's End 1 with an "S" rank.
Fire/Water Key	Beginning 2 with an "A" rank.
Fuel Canteen	Mansion Incident 3 with an "S" rank.
G-Virus	Fourth Survivor with an "A" rank.
Great Eagle/Wolf Medal	Mansion Incident 1 under the Hard difficulty with an "S" rank.
Hex Crank	Nightmare 1 with an "S" rank.
Ink Ribbon	Umbrella's End 3 with an "S" rank.
Joint N/S Plug	Raccoon's Destruction 2 with an "A" rank.
King/Knight/Bishop/Rook Plug	Dark Legacy 1 with an "A" rank.
Last Book, Vol. 1/2	Nightmare 2 with an "A" rank.
Lighter/Lockpick	Mansion Incident 1 under the Normal difficulty with an "A" rank.
Lighter Fluid	Raccoon's Destruction 2 with an "S" rank.
Medium Base	Dark Legacy 1 with an "S" rank.
Microfilm A/B	Beginning 2 with an "S" rank.
Mixing Set	Train Derailment 1 with an "A" rank.
MO Disk	Mansion Incident 3 with an "A" rank.
Motherboard	Beginning 1 with an "A" rank.
Picture (Ada and John)	Death's Door with an "A" rank.
Plastic Bomb/Detonator	Umbrella's End 1 with an "A" rank.
Relief of Discipline/Obedience/Unity	Train Derailment 2 with an "S" rank.
S.T.A.R.S. Card (Brad)	Death's Door with an "S" rank.
S.T.A.R.S. Card (Jill)	Raccoon's Destruction 1 with an "A" rank.
Slide Cartridge	Rebirth 1 with an "S" rank.
Spade/Diamond/Club/Heart Key	Dark Legacy 2 with an "A" rank.
Square Crank	Umbrella's End 2 with an "S" rank.
Statue of Good/Evil	Train Derailment 2 with an "A" rank.
Sterilizing Agent	Train Derailment 3 with an "S" rank.
Sun/Star/Moon Crest	Mansion Incident 2 with an "A" rank.
V-Jolt	Mansion Incident 2 with an "S" rank.
Vaccine	Umbrella's End 3 with an "A" rank.
Valve Handle	Beginning 1 with an "S" rank.