There are two forms of observation: passive and active.
Human senses usually receive information passively. For example, seeing objects or hearing sounds occurs because the target emits light or sound waves, which reach the observer's eyes or ears.
In such cases, as long as the observer hides themselves well, they often won't be exposed.
Active observation, on the other hand, requires the observer to send out signals themselves, like radar scanning, or the echolocation of bats and dolphins.
Take a simple example: in a dark cave, to see things clearly, you have to turn on a flashlight to illuminate the surroundings. But when you turn on the flashlight, things in the dark will also notice you.
When a cultivator expands their divine sense to investigate, it is also a form of active observation. Although ordinary people might not feel it, sorcerers, especially those in a state of alert, can detect this disturbance.
