„Good“ triggers were deliberately engineered to be unpredictable to make defusing harder and so you cannot assume the the area is safe just because the bombing has stopped.
The most notorious example is a time-delayed chemical fuse, it uses a slow chemical reaction to detonate the bomb a few days later, as a form of psychological warfare. If the bomb didn't explode as designed, it means the chemical somehow got stuck, extremely dangerous to defuse, no matter how much prior knowledge one has.