ALL of the screenshot keys/shortcuts in Windows 8.

Sporadically when I’m using my Surface Pro 2, Dropbox helpfully lets me know that a screenshot has just been taken and added to my Dropbox/Screenshots folder. What is causing my computer to take screenshots? Googling the problem doesn’t yield anything except numerous different sites telling you the main way to take screenshots on the SP2 (IIRC press the Windows button plus Volume down on the tablet). I know for sure that I’m not doing that.

I considered that I might have some poorly designed yet insidious spyware on my machine, but MalwareBytes and Spybot come back as clean. I think it’s more likely that I’m accidentally hitting some arcane combination of keys that takes a screenshot. Evidence in favor of this is that the Shift and Ctrl buttons frequently stick.

Or maybe I’m entirely off base. Any ideas?