The tiny vent on the top of the electrical cabinet is the central air system filter vent.
It may have just been venting an accumulation of dirt as that cabinet doesn't normally draw air from the engine room, rather it pressurizes the electrical cabinet and in some units the engine room to keep dirty air out. The central air system draws air in from the large
square grill behind the cab and a high speed fan of sorts spins and ejects the dirt out that small square hole in the roof, and sends the clean air to the electrical cabinet and the traction motors. On the walkway behind the cab on the fireman's side is a tall box that contains a secondary filter that filters the air a second time before it's sent into the electrical cabinet. (When EMD first introduced this system, the second filter wasn't used, but was added later by most railroads and soon became standard.) It is also possible, if the unit was smoking heavily and not going real fast or in a cross wind, it could have been sucking some of the exhaust in the central air intake and ejecting it out the dirt vent in the roof. Just some thoughts, but it is nearly impossible for electrical cabinet smoke to come out that vent as that system blows air into the electrical system.
(Some railroads experimented with placing shields over the central air intake to keep fine snow and exhaust soot out of the central air system intake.)