Unless you are a terrorist, spy or have documents worth millions. I highly doubt someone is lurking about, watching your every move and going to go through the effort required to retrieve data from the memory buffer of a scanner. Plus you could scan a couple pages of nonsense afterwards and fill the buffer.
That's some serious data retrieval. It would take someone who really knows their ****** when it comes to data recovery.
Sure it's possible. The scanner could be rigged or some other nefarious circumstances occur. You could also get struck by a meteor or mauled by a Bengal Tiger when you step outside. It's just extremely unlikely.
Odds are vastly higher that someone would mug you on the way to or from one of those places and steal whatever is on you. Another much more likely occurrence is absentmindedly leaving the documents in the scanner or some other public place.