Agreed, it would not be possible for ScotRail to operate a three car Class 158 in normal passenger use
However, single cars have been moved around although very rare with the additional car normally marshalled such that it is hauled at the rear of the unit
Normally if one of the cars requires to visit the Lathe at Shields the entire unit goes
This therefore suggests the stored car is remaining at Haymarket, the spare is either en-route to Glasgow Works or is at Eastfield for easier road access to/from accident damage repair
From memory there is still an Inverness based Class 158 undergoing repair due to level crossing collision damage, and that would make sense as only car was severely damaged, in which case this may be this car making its way back after repair
Corkerhill perform the same splitting of units, the stored unit is placed into the short siding visible from the M77