My guess the HVAC Tech’s didn’t get along with the Plumbers. The pictures show the drain from the toilet in a 1st floor 1/2 bath running through a cold air return. I haven’t run across this before. Home was built in 1998. Any suggestions?
If it leaks, it’s a real “crappy” situation… 
Unless this is commercial, codes never thought about it yet.
M has priority over P so if you call it, it’s the plumbers problem.
What about the “A” that designed it?? 
I thought about mentioning to seal around the PVC, but sucking a little basement air would be a whole lot better that plugging a potential water leak so I left it alone.
It’s uninspectable. How do you know the floor isn’t rotted? Disclaim it.
The toilet sat on top of an air return??? I can’t picture what the bathroom floor looks like. Got a pic?
The floor was tile and the toilet was tight. No visual evidence of any problems. Noted in the report as acceptable with a note as unusual installation and sub-floor was not visible.
May not be the smartest, but I think it is allowed.