What happened here?

[tevo]

It was a super dark, clear night last night so I decided I would try some Milky Way shots with the x100s. After a few exposures I noticed what I thought was a light leak in the bottom left of the frame. I made sure my flashlight was off and I tried again, same thing. I need to stress that it was PITCH BLACK outside. There's no way light could have gotten in. Furthermore, my exposure was the same. I had taken a few 30" exposures before this one, and these cloud things started to appear. What are they? Was it due to the sensor heating up maybe? GHOSTS?! This image is SOOC.

 

[The_Traveler]

I wouldn't be surprised if this was light reflection from clouds.
It isn't just in lower left

 

[Msteelio91]

Definitely looks like that to me, clouds picking up the reflections

 

[Eclectix]

Looks like clouds. It may look black to you, but if there is a city within 50 miles they will reflect a little of that light and your camera will see it.

 

[D7K]

Agreed looks like reflected light pollution