Oh my gosh, I just posted a thread last night about my snake doing this. She opened her mouth three times and made what to me sounded like a type of hiss. It was right after eating for mine. She was looking right at me when she did it, so I ultimately look it as a series of "back off" hisses for no apparent reason, but now that I think about it, defensive hissing is generally with the mouth closed, not wide open.
Anyway, all of that to say, I am as stumped as you, but my snake also just did this and she is currently doing her routine midnight tank exploration, so she is just fine. Can't tell you why your snake did it, but I can tell you she isn't the only one.
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