I get a lot of these TARTNxx flights, they normally display with Company = Royal Air Force, and the TunisAir logo. In fact, the RAF use so many different callsigns, you just get used to ignoring the logo for these, and look at the RAF flag and Company.
Other RAF callsigns include - FAGINxx, RRRxxxx, NATOxx, WADxxx, etc., and I'm gradually adding appropriate logos for each of these.