Are you running v2.0 or v3.0 beta. Looking at port 7879, I also see aircraft without callsigns/FlightIDs.
EG:
<MODESMESSAGE>
<DATETIME>20090702103928</DATETIME>
<MODES>A8BD53</MODES>
<ALTITUDE>10700</ALTITUDE>
<VRATE>0</VRATE>
<AIRSPEED>000</AIRSPEED>
<LATITUDE>0</LATITUDE>
<LONGITUDE>0</LONGITUDE>
<SQUAWK>000</SQUAWK>
</MODESMESSAGE>