Hacking through wireless NIC?

My users kept bugging about someone peeping into their notebooks via their wireless NIC because they get the message "someone is logged on to your network" when they were shutting down their notebooks at cafe, restaurants etc. Are there tools I could use to stop these and can someone logon to another's station via wireless NIC? I thought it's only possible if I set it as an adhoc network i.e. peer to peer.