Is it possible to detect collisions without a rigidody?

Questions & Answers
1

If you still using collider to make it happen,
Then you still need rigidbody

Another way is using a script, like:
if position object A is near position object B then object A is destroyed

What is the reason you dont use rigidbody anyway?

2

You can detect collisions without a rigidbody.
However, if you haven’t got a rigidbody attached to the object then you won’t be able to apply force to it through script and you will have to use something like this e.g.:

transform.position += transform.forward * Time.deltaTime * speed;

There is nothing wrong with using this technique instead of applying force, but the draw back is that, while the collision will be detected (if the object has a collider component) it won’t actually “collide” with the object…
The difference being that you will be able to detect a collision through code e.g.:

function OnCollisionEnter() {
   //collision detected
}

But the physics engine won’t calculate the collision for you, because the object has no rigidbody that makes it run on unitys physics.

So to answer your question; Yes it is possible to detect collisions, but without a rigidbody the physics engine won’t make the object actually react to the collision.