UNID geosat Unknown 121118 is probably the classified SIGINT Mercury 1 (94-054A)
I initially thought it would probably be a commercial satellite that had been recently relocated. However, as USSTRATCOM has still not identified the object, we are beginning to suspect it is a classified object, and we have some idea of its identity.
Ted Molczan pointed out that the 8.44 degree inclination is similar to that of Mercury 1 (94-054A). The brightness of the object is similar too. Mercury 1 was last located over the western Atlantic near 43 W, and had not been observed for over two months. Peter Wakelin imaged its former position on November 21 and could not find it. So there is a good chance it has been moved, and is my Unknown 121118, now located at 48 E.
The Mercury (also known as 'Advanced Vortex' ) geostationary satellites are classified US military SIGINT ('eavesdropping') satellites. Two were launched during the 1990-ies: the launch of a third one failed when the rocket booster malfunctioned, destroying the satellite. Mercury 1 (94-054A) launched on 27 August 1994 was the first. Given that it now appears to have been repositioned and is station-keeping at 48 E, is appears to be still operational, 18 years after it was launched.
Why it has been repositioned over 48 E, somewhere within the last two months (and probably near the more recent part of that timespan) is a matter of speculation. Maybe it is monitoring communications on the Syria-Turkey border; maybe it is listening in on Iran; or maybe it is monitoring communications of Somalian pirates near the Horn of Africa.
In June this year I imaged Mercury 1's sister ship Mercury 2 (see here), which was of a similar brightness as 'Unknown 121118'. That object at that time seemed headed for a graveyard orbit and hasn't been observed for a while. It has a slightly larger orbital inclination than Mercury 1 / Unknown 121118.