Paul, On my last trip to Trump Marina I saw a single card laying on top of a machine. I asked the guy sitting at the machine if it was his. He did not respond, not even a grunt. I picked up the card and kept walking. About a minute later I was stopped by security and asked if I had "pick up a card by mistake". I pulled out a stack of cards from my shirt pocket and handed that particular card to the security guard.
So the answer is YES, you can be stopped by security for doing this.