This is one of those fun stories that you want to believe in, especially because it sounds like it could happen to any of us! What is it? 

Photographer Zach Alan just went to Hawaii to visit Volcano National Park. While there, he shot a few photos of his subject. At the airport on their way back home, he was going through the files and he saw something interesting. 

He shot on burst for a series and so each frame was milliseconds apart from each other. You can clearly see that in one frame in the video below that there was absolutely nothing in the fog above, then suddenly you see something come into frame. 
 

The great part is he gave us the link to download the raw files below: 

Link: https://drive.google.com/drive/folders/1ELmUA8CoVr-pP3lE9fP3dKE_fVvSsOGo

Of course, I had to download one and pull the information to see what it was.

Judging from how the detail looks, I had a few thoughts:

1. You don't "see" effects of motion blur, it does appear solid in how the colors span horizontally. When you open it up in Photoshop you see that it's something solid. 

2. Even if it is shot at 1/30, you could see based on how fast he was shooting, something had to be substantially fast in order to be captured in between those frames. 

3. If it was sensor dust, it would have been captured in the previous frame, and it would mostly be circular or cylindrical. 

We'll never know, but it's fun to think about!