The problem is that there is no a specific program/service/daemon to check when a new wireless network appear/disappear in/from the range of the wireless antenna as far as I know (in fact there is airodump-ng and maybe others, but this conflicts with Network Manager).

As alternative, you can scan for wifi networks from time to time, using iwlist tool.

Note: In tests iwlist tool may only work with Atheros, Intel or Broadcom cards.

These being said, the following script can do what you wish (I named it wifidetect):

#!/bin/bash



#wifidetect - Alert when new wireless 



#Licensed under the standard MIT license:

#Copyright 2013 Radu Rădeanu (http://askubuntu.com/users/147044/).

#Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

#The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

#THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE



DISPLAY=:0.0

#change 'username' with your username

HOME=/home/username/

XAUTHORITY=$HOME/.Xauthority

export DISPLAY XAUTHORITY HOME



#check if you run the script as root

if [ "$(whoami)" != "root" ]; then

    notify-send -i error "You must to run $(basename $0) script as root."

    echo "You must to run $(basename $0) script as root."

    exit

fi



#check if beep is installed

if [ ! -n "$(dpkg -s beep 2>/dev/null | grep 'Status: install ok installed')" ]; then

    notify-send -i error "The package 'beep' must to be installed before to run $(basename $0)." "Use 'sudo apt-get install beep' command in terminal to install it."

    echo -e "The package 'beep' must to be installed before to run $(basename $0)

Use 'sudo apt-get install beep' command in terminal to install it."

    exit

fi



[ -f /tmp/networks ] || > /tmp/networks



#scan for networks - need root privileges

#change 'wlan0' with your interface name of your wireless network card; you can find it using 'ls /sys/class/net' command

sudo iwlist wlan0 scan | grep -E "Address|ESSID" |  awk 'BEGIN {FS=": |""} {print $2}' | sed 'N;s/

/ /' | sort > /tmp/new_networks



#check if there are new networks

new_networks=$(comm -1 -3 /tmp/networks /tmp/new_networks)

new_essids=$(echo $new_networks | sed "s/..:..:..:..:..:..//g" | sed "s/  /, /g")



if [ -n "$new_networks" ]; then   #if there are new networks

    #send a graphical notification

    notify-send -i /usr/share/app-install/icons/wifi-radar.svg "New network(s) detected:" "$new_essids"

    #send a sound notification

    pcspkr_on=$(lsmod | grep pcspkr)

    if [ -n "$pcspkr_on" ]; then 

        beep

    else 

        sudo modprobe pcspkr

        beep

        sudo modprobe -r pcspkr

    fi



    #change the old list of networks with the list containing new networks

    #the networks that were detected in the previous scan and wern't detected this time will be removed from the list

    cat /tmp/new_networks > /tmp/networks



    #if you want that only the new networks detected (who were not detected in previous scans) to be added to list, comment the previous line and and uncomment the following three

    #cat /tmp/networks > /tmp/tmp_networks

    #cat /tmp/new_networks >> /tmp/tmp_networks

    #cat /tmp/tmp_networks | sort | uniq > /tmp/networks

fi

Note: You have to change in script username with your user name and wlan0 with your interface name of your wireless network card; you can find it using 'ls /sys/class/net' command.

Don't forget to make the script executable, using the following command in terminal:

chmod +x /path/to/scripts/wifidetect

Finally, edit the crontab entries for root using sudo crontab -e command by adding the following line:

*/1 * * * * /path/to/scripts/wifidetect

I have set the cron job for every minute, but you can change as you wish or as you think is better. See http://en.wikipedia.org/wiki/Cron in this sense.