Hi Ian,
The comprehensive reply given buy Trubb on the mode-s forum yesterday gives you all you need to nknow.
Hex 249249 (or octal 11111111) is often used by the USN/USMC as a
default code. The flight ID "GOTO FMS" is almost always used by USN E-6s
which, from your description, is clearly what you saw.
For info USN E-6B 162782 went into Stuttgart on the 5th & departed
yesterday [credit 'STR Forum' (see
http://www.strforum.de/viewtopic.php?f=9&t=10238)] and on both occasions
used hex code 249249. Looking at Albert's site that code was picked up
in Portugal & Spain yesterday afternoon, and again today, so it may have
recovered to NAS Rota last night.
Given that it is a default code it could be in use by more than one E-6
of course, but all things considered it's very likely that the one you
saw today and the one in Stuttgart on the 5/6th are the same machine.