As said you pretty much have to almost touch the pad for it to work. I wouldn't worry about somebody standing behind you with a high tech scanner, the technology just doesn't work that way. Although there are a quite a few people in this world that are more than happy to make misleading claims and take your money for a protective sleeve.
And seriously, what's the concern? It's a Visa. If they somehow steal your card you get the money back.
My unsolicited advice... Login to your visa card website or call the card company using the phone number on the back of the card and ask for assistance. Setup an alert that sends you a text any time the card is charged. Set the dollar amount for the alert as low as possible (typically $1). Then go about your day and don't give this another thought.
I have this setup and I often get that text while standing at the register waiting for my receipt to finish printing. If not I definitely get the text before I get to my car.
If you ever get a text when you know you didn't just use the card you can be concerned. Until you figure out that it was your monthly Netflix renewal. Yep even after many years I still get caught by that one every month.