It is true that a person can suddenly disappear like a shadow in some cases, but this is very rare. Generally speaking, it is impossible for a person to suddenly and completely disappear without any warning.
First of all, from the point of view of physics, human beings are material entities and cannot simply disappear like shadows. Even in very special cases, such as phenomena such as black holes or quantum entanglement, it is impossible for humans to disappear completely like a shadow.
A black hole is an extremely dense celestial body, and its gravitational pull is so strong that no matter can escape from it, but the black hole itself is not a "hole", but a point formed by the aggregation of matter.
Quantum entanglement is a quantum mechanical phenomenon that causes an entanglement between two or more particles that makes their states indescribable, but this phenomenon does not apply to humans.
Secondly, from a biological point of view, humans are complex organisms and cannot suddenly disappear like a shadow. Even in the case of death, humans go through a gradual decay process that eventually decomposes into basic chemical elements.
However, there are some special situations in which humans may experience situations that resemble "vanishing". For example, in a magic show, some magicians may use tricks and props to create the illusion of a human suddenly disappearing.
In addition, in some sci-fi** or movies, humans may also experience some supernatural events that cause them to suddenly disappear or appear. In short, it is impossible for humans to suddenly disappear like a shadow. However, in some special cases, you may experience some situations similar to "disappearing".