Emergency-carryall for units that can't exit
Theres actually an additional "emergency"-case apart from the "insurance-harvester" where an external carryall appears on the map and drops a harvester on the refinery ... and that is when you completely block off a production-building and a unit can't exit out ...
a emergency-carryall gets triggered and (appears from the edge of the screen or map) grabs the (nonvisible) finished unit from the enclosured building and places it on the next free cell
Not necessarily the next free cell though, I've noticed them being placed quite far away:
both of those heavy attack quads were just produced at the light factories.
these are from the Dune 2 Android app, but I think it's supposed to be as close to the original as possible