Two survivors scavenging a Residence
Scavenging, a.k.a "looting" or "searching", is a feature in all installments of The Last Stand series.
The Last Stand
Weapons and survivors can be found by putting hours into searching for either of them during the day. Finding weapons is determined by a bar which is filled up as the player puts hours into searching for them. Finding survivors is random. Note that survivors can be killed while searching.
The Last Stand 2
In this game, weapons, traps, survivors and supplies are found by searching buildings on the town maps. Each building takes a certain amount of hours to search, of which the player has a limited amount per day. Finding anything is random, though certain buildings have a higher chance of having a specific type of item. Survivors can no longer be killed while searching.
The Last Stand: Union City
Here, weapons, ammunition, food, drink, skill books, clothing, medical supplies, cash, junk, keys and several quest items can be found inside various containers, ranging from cupboards to footlockers, scattered throughout Union City. Items found inside a container that are not taken immediately will remain in the container and can be taken later. Searching a container grants XP to the player, which is affected both by their Smarts skill and their current level.
Some containers are locked, requiring the player to either pick the lock or find a key in order to search them. Most locked containers can only be unlocked by one or the other, some can be unlocked with either method. Note that should the player fail when using the Force Lock option in the minigame, the container is permanently locked and cannot be searched.
What can be found inside each container is preset, though there is a chance of finding additional items (marked with a star and called Lucky Finds) should the player get lucky. The Luck stat increases the chance of getting Lucky Finds while the Searching skill decreases the amount of junk that is found.
See the relevant location pages from here for a list of what containers are present in each location.
The Last Stand: Dead Zone
Similarly to TLS:UC, weapons, gear, resources, components and junk can be found in various containers in locations throughout the Dead Zone. Searching a container provides XP to all survivors on the mission (and the Leader, even if he/she is not present), determined by the level of the mission.
However, unlike TLS:UC, the items found are random, though the type of container and/or location being searched influences the results. The game randomly rolls per level for what items can be obtained, with it rolling for lower levels should the previous level not have anything, but did not get a result where nothing further can be obtained from that container.
Any survivor can search containers, however, the time it takes to do so is dependent on their class and whether they have a Hand Wrap/Gloves/Scavenger Vest gear item equipped. Containers that do not contain any items take a shorter amount of time to search, but still give XP.
Players can also scavenge from other player's compounds, which is more commonly known as a raid. More information is available in said article.
All resources and items found from scavenging are immediately available at your compound as soon as the mission is completed, unless it was automated, in which case, it will be available once the survivors have returned to the compound. However, if all survivors on that mission were incapacitated, all scavenged items and/or resources will be lost.