Explanation?

it's half a unit of A stock basically.

Some cabs in the middle of a train were refurbed for the Chalfont - Chesham and also the ELL. Some weren't and were just left as a huge shunting cabinet. They have no radio and some don't have equipment because they have been 'robbed'

it's odd to see one on the tracks though.

I'm no expert, but most LU refurbed stock has been similarly treated. There are middle cabs that are out of use on the 72ts amongst others I know of. On the D78s they are doing all of the middle cabs for 'fleet flexibility'