Is there a word for 'what a witness sees'?

A witness sees an occurrence

• "something that happens"

an action.

• "a thing done"

or an incident, as mentioned in a comment.

• "an unexpected and usually unpleasant thing that happens"

Although a witness does observe, an observer does not always witness. According to Hindu philosophy, a witness is described as one "who is here to observe and enjoy this world of objects, unending perceptions and wonderful creations, without becoming attached to them" (Jayaram V, Hinduwebsite.com). This suggests a personal relationship between the observer and that which is observed or witnessed.

As such, a witness not only observes, but perceives, which suggests awareness and appreciation of what is being seen. So, perhaps Perception would mean more than Observation in your context.

One could ask a person if they witnessed the event.