Im curious as well. Same thing happened to my vortex. My guess was internal coating coming off? 
Interesting you say that because I read on another site where a guys Vortex did this and then they sent him a new one and that one did it as well. Mines not a Vortex though, just to clarify. I thought about the internal paint possibly bubbling or something but it was only 70 degrees yesterday and I wouldve figured that would only happen in extreme temps, but I really have no idea.
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