Stray packets can still be picked-up by a dedicated hacker
Q: How do I know if people are listening to my wireless network traffic?
A: You don't. If you've planned out your wireless deployment properly there's very little usable traffic escaping the building, and all of your traffic is encrypted. But a dedicated hacker with the proper equipment can still pick up some stray packets. You should assume that there's always some miscreant listening and protect your wireless network with that in mind.