Your are quite right about my assumptions....
Having seen a furnace run without a return in an attic a few times, I am
basing it on that...
MOST of the time, attic furnaces have Return grill air filters so the
homeowner doesn't have to go into the attic, so my assumption about
unfiltered air is based on that...
As for stuff in the air... I spend a LOT of time in attics, and there is a
LOT of stuff flying arround in attic air, none of which I want in my lungs
or in my AC unit.
The unit I saw recently the return was FULL of insulation, as windy days
stirred it up just a bit, so little by little the unit vacumed it up, into
the rteturn, the fan, the exchanger and the coils... really really ugly.
I have seen the same with units in the basement when there was no cover on
the filter, it sucked air from the basement PAST the filter...
In the worst case, when I turned on the heat, there was enough stuff on the
heat exchanger that it smoked enough to set off the smoke detectors... and
that was in a brand new house, not even occupied yet...
I am paid to be paranoid... and what I often see makes me even more so... I
rely on the "Professionals" like you to use your training, experteese and
experience to determine if it "just that damed Paranoid inspector" or if
there is a real problem. I only see the house for a few hours, you guys
get to service the unit for its usefull (ha!) life.... What happens most
of the time is the builder sends his puchout guy up with a damp rag and
maybe a shop vac.... does a really good job cleaning the unit.... notice I
didn't mention a screwdriver...