Everything that can be searched counts as a container.
At times you will have side quests available that task you with finding a certain amount of items; like the Handyman's Assistant side quest where you have to find first 6 parts, and then later 5 parts for the mechanic. These are searchable containers that only show up when the side quest is active, and in a few cases, make a previously searched container searchable again. I'm fairly certain all the containers you need to search for side quest parts counts towards this achievement.
I got this early in Chapter 2, just before reaching the third area "Sand" in Shadowlair when you "Escape to the poor neighborhoods" during the main quest "Back Home".