This started happening after a recent update.
When I reboot the network-manager-applet in the system tray no longer shows my wifi networks, although it does actually connect to my wifi network.
In order for the networks to show and the signal level icon to show I need to reinstall the applet at the command line:
sudo pacman -S network-manager-applet
Then I log out and log back in and all is well until my next reboot.