Items such as (2nd) weapons and the jerry can (Gas Canister) will appear in the character's inventory after being dropped from "in hands" to the ground using the red X.

The Gas Canister will appear at the top of the inventory screen with items such as hats, gloves, shoes, etc.

If a character has a primary weapon on its back already, and picks up another primary weapon into the hands and drops it using the red X, then either immediately or after a few (up to 10) seconds, it will disappear from the ground and appear in the character's inventory at the bottom (below the backpack).