Just found two mobile devices which are not detected.
user agents:
Mozilla/4.0 (PSP (PlayStation Portable); 2.00)
and
Mozilla/5.0 (en-us) AppleWebKit/525.13 (KHTML, like Gecko; Google Wireless Transcoder) Version/3.1 Safari/525.13
neither are counted as visits of any kind. i.e. the are completely ignored so don’t show in stats at all.
the first one runs javascript. The second one doesn’t but I can see the nonscript call to piwik.php
edit:
The second one is IP 74.125.16.66 which is google. That may have been me testing in mobile search on desktop browser. But the first one was definitely not me.