I've seen it a couple of times now: there are pics in my library of me performing palm control.
That proves that sometimes Spark is making selfies on its own initiative without me performing the selfie gesture.
After that I saw that sometimes Spark is hoovering in gesture mode and all of a sudden the front LED's are flashing red rapidly (and so it is taking a selfie out of the blue).

More people that have noticed this behaviour?

Same here... 3 or 4 photos were taken without my gesture in palm control.

Yes, I have a few pictures like that as well.  Most seem to be at the exact moment I put up my hand for palm control.  Like it can't distinguish between my palm and the selfie gesture.  I chalked it up to distance from palm to spark not being quite right or not being deliberate enough in my actions.  Not sure though.  

Be carful cause I tried the selfie gesture and the spark to off sideways and crashed into my porch.