The texture on the underside of the shade of the hanging spotlight (HangingSpot in the Object Heirarchy) is not illuminated, but clearly it should be (compare it with HangingSpotFlush).
Fixing this oversight was quite simple, and you can download
the fixed version of the model Here
I have also remade the light shade so that it uses fewer polys.
The fix works in both Thief 2 and Thief 1 because all the textures use
the original Thief palette.
The image below shows the difference between the original
(left) and the new (right):
Create a folder called obj
in your Thief installation folder (unless it already exists) (e.g. Thief2\obj\)
and extract into it the file ELECSPOT.bin
(For Thief 2 you can get nicer versions of the lamp-glass texture
Here)