Have you ever used an old coffee can at home to clean paint brushes? You probably didn’t label that can to show it contained solvent (but you didn’t leave it full of solvent and put it on the shelf, right?)
Construction site workers, unless they’ve been working for fly by night outfits, have extensive safety training. They typically know, for example, to label all containers (including open “temporary use” cans).
When a company opens a new plant, they typically staff it with “green” local hires. Consequently, many new hires are on the job but their basic safety training is still incomplete. Training out bad habits that people bring with them takes time. One such habit is using an unlabeled container.
The unlabeled container problem does arise on construction sites, but it’s especially pervasive at new plants. Don’t open containers not acquired through your normal process or not associated with your job.