A lot of unused contents are still present in the game files. I try to gather different informations on this thread.
I used different topics on this forum and videos from randomizer mod to get some informations about hidden contents.
Other source : tcrf Hitman 2/Unused Inventory Thumbnails - The Cutting Room Floor
All EDIT: Added pictures (Source : https://imgur.com/a/kgYJpHT; https://www.reddit.com/r/HiTMAN/comments/js3blr/a_feature_cut_from_hitman_12_item_concealment_in/ and YouTube video about Randomizer mod)
Unused items present in game files:
ICA19 (without silencer)
Bartoli 12G Short (shotgun)
Fusil X2000 Stealth (assault rifle)
TAC-4 S/A (colored with black and blue)
Sieger 300 (but looks like a Kazo TRG)
ICA19 Goldballer

Several unnamed weapons
Antique sedative syringe (poison)
Sedative syringe MKII (poison)
Sedative poison gum (poison)
Emetic poison gum (poison)
Lethal poison gum (poison)
Proximity sedative gas device
Proximity emetic gas device
Remote sedative gas device (tool that knocks out NPC)
Remote cure gas medium range device (knocks out NPC)
Remote cure gas small range device (knocks out NPC)
Proximity cure gas medium gas range
Cure coated coin (knock out when recovered)
Breaching charge MKII (explosive)
Ceremonial staff (melee?)
Eiffel tower knife (melee)
Meat fork (melee?)
Old rusty hairpin (lockpick?)
Power bait (?)
Anti-virus large (?)
Kalmer 3 - Cure gun (?)
Brocken drumstick (?)
Honey jar (?)
Soap
Conceal object in it
Crate kitchenware
Crate tomatoes
Golden briefcase (apparently not golden)
Flower bouquet (where you can hide object in it - like a briefcase)
Jerry can (pour a oil spill that can be light up)
Video showing the animation with the jerry can and how it works : HITMAN 2 | Future stuff | Fuel - YouTube
Magic watch (seems to change time - makes the game goes quicker)