Could the camera record the knocks but show nothing?

It shows that a sound reached the camera’s microphone, not that the camera captured its source. If the camera’s view is narrow, the cause could be just outside the frame, obscured, or too quick or small to notice. Check the footage before and after the knocks, and whether the camera records sound and video in sync.

I’d start with ordinary explanations and a careful look around in daylight. A loose shutter or trim, a branch, or a vibration traveling through the building can make a sharp knock without anything appearing in the camera view. Check nearby windows and fittings, and see whether the sound happens again under similar conditions.

Nothing in the clip establishes that it was a ghost. It also doesn’t rule out a cause the camera couldn’t see. Save the original recording, note the time, and compare it with other camera views or recordings if available. If there’s no visible source, the honest answer may simply be that the clip doesn’t identify what made the sound.