Spoiler alert: just read the original story if you want to be led through all the clues, one by one, as LiveScience tries to eliminate the supernatural. Otherwise, here’s their conclusion:

One obvious answer, prematurely dismissed by Peterson and others, is almost certainly the correct one: the ghost is a bug. A spider or insect wandered onto the camera; that’s why it was out of focus, why it seemed to glow, why it didn’t interact with anything in the room, and why it only appeared on one camera. It’s true that a moving light wouldn’t activate the camera, as they are sensitive to motion, not light. But it was the bug’s movement that triggered the sensor and started the recording.

This gives new meaning to a surveillance bug.