That's thing, it's /sbin/ifconfig and set as world executable, both are 100755 can't actually see any difference. 
antares@antares-ThinkPad-X1:~$ ls -la /sbin/ifconfig
-rwxr-xr-x 1 root root 78960 jan 10 2017 /sbin/ifconfig
there's the extra line for the admin group in sudoers that does the trick, methinks:
%admin ALL=(ALL) ALL
