I'm using Debian 8 stretch and after initial installation all the network icons worked just fine. After upgrading to latest version via apt some icons went missing (they're just blank). Some version information:

KDE Plasma Version: 5.4.3
Qt Version: 5.5.1
Kernel Version: 4.2.0-1-amd64

So the icons for Mobile Broadband are missing. Also the settings icon (wrench) was missing but it came back after I deleted ~/.cache/plasma* and icon-cache.kcache. The "airplane mode" icon changed too. It used to be an airplane symbol but now it looks more like the GPS icon. Wired Ethernet icon is missing if it's not connected. When connected the icon is there. See the image below:



I've been trying to find out where the icons are to check if they exist but haven't found them. I'm using the Breeze Dark theme with Oxygen Icon theme. However the system icon theme does not affect the Plasma icons. Please let ke now if you need any more debug information.