Places to Find Them
Most loots and drops are found either when you kill a zombie and get one (very rarely), or a hidden room with zombies and loots inside. All maps have drops and can be used to the player's potential, for instance in the level Vaccine, immediately right and above the player's spawn point, there appears to be a stack of papers on the ground by a wall and can be shot at to reveal a hidden control room with a loot/drop inside it, as well as entering the door of the the crashed Airplane in the airport. Furthermore, the level Meltdown has both a hidden room and a secret cave with lots of rewards.
Loots and drops can be extremely helpful when low on health and you find a medkit, or when you pick up several grenades to throw instead of buying them. All in all, loots and drops can be found by the clever player and used to your advantage; however, higher levels may simply ignore all loots except high-level strongboxes.
The Christmas version of SAS 4 has Christmas decorations that can be destroyed. These decorations are pretty tough, but they always drop a strongbox.