My neighbours next door have a dlink access point which is set to factory defaults. they are not tech savvy and i could easily connect to their network and use their internet connection at will - if i so desired.
the only way they would know i was on the network was looking at the DHCP table on the access point (impossible given their lack of techspertise) or they could do a scan of the subnet by doing a broadcast ping or using a subnet scanner.
again this is unlikey given their level of knowledge.
so how about an LCD display on the access point which tells them how many clients are connected and updates with an audible and visual signal when a new client connects ?
just a thought,