Placement of your wireless access points is the first layer of security for your wireless network. If your wireless access points can be physically touched by a passerby you have major problems.
The easiest way to hack a wireless access point is walk up to it and press the reset button. This will make it default back to an open network and poof you have keys to the kingdom
When placing your wireless access point the only thing you should be able to see is maybe the antenna. You can purchase access point antennas that look like ceiling panels or fire alarms.
If a hacker is in your building probing your wireless network he is going to look for access points. If he can determine the make and model of your access point he can search the Internet for default passwords and vulnerabilities for that access point.
Finally when placing your wireless antennas outdoors try and make them blend into the building as much as possible. You should use a wireless directional antenna that is flat against a wall rather than a Yagi antenna which screams look at me.
Wireless access points can be hacked in 20 seconds or less if they are mounted in the wrong place. The most secure wireless network is the one nobody knows about.