I Have been sleeping so it seems...
There was already some confusion in March about this frame when it visited Brussels as well.
Seems 05-5141 frame is showing wrong hexcode (of 05-5151) since at least January this year.
See here for most recent log
See here although not sure if these were mode-s logs or visual
See here last March at Brussel
See here were it indeed was a March callsign, so should have rang some bells
In December 2013 05-5151 was logged (mode-s or visual?) with TREK callsign, which would fit.
See here
Therefore I draw the conclusion that "something" happend with both 05-5141 & 05-5151 beginning this year, and even not sure if both fly with AE1455 Hexcode.
Safe to say to at least doubt Mode-s logs of 05-5151 from January 2014 onwards.
Thanks to Mk1 Eyeball sightings so I can update my sbs database.. Many thanks.