Did a condo inspection, today. Newer building (2007) multiple unit condo. The unit was, basicly, a studio with a bedroom area partitioned off.
The unit had a utility room with a cat 4 high efficiency furnace. It was a sealed combustion unit and was properly vented, with both combustion air and combustion gas flues to the exterior.
It had no return ductwork. The utility closet (furnace only in there, common water heater) had a solid door and an 18 x 36" return register over the door. The return intake on the furnace had the filter in place, but no ductwork attached.
I checked with my local HVAC expert, the VP of operations for the local gas utility, and after asking some questions and thinking about it, he sair it was OK. Essentially, they were using the utility closet as a return plenum. Since the furnace was a sealed combustion unit and properly vented, there was no need for return ductwork.
It was a cheap build, but it was technically OK.