On this Forced Air/heating system I normally see one or
two ceiling filters ( on a 2 story, 2400 sqft house ).
The grill flips down and you can change the filters.
On this house I seen a grill without a filter( on the ceiling). Its screwed
down…no easy access. Would this be the return air ?
Looks like a return. Filter at air handler?
If you did not locate a filter then that would be the location.
I would imagine the blower compartment looked a little dusty in that case.
The returen filter if not installed in the wall where the grill is, is generally located at the air intake to the furnace blower system. look for a slider cap you can remove and then remove the filter. *look for the size of the filter, written on the duct work. The filter will be close at hand.
You can use the ceiling grill or the return duct at the air handler or both. If the ceiling grill does not have the latches (screws only), then the air handler location is best. Or simply have the ceiling grill upgraded to a hinged grill, which is more convenient than having to access the attic.
Just did a large house yesterday, every room has a very small return air grill, screwed down, no filter, with a very large filtered return air at the unit located in the hall. Took the house (vacant) about half the time to get to the temperature called for at the thermostat, no hot spots. Now, the attic was full of HVAC ducting but it worked like a dream. I typically see these in homes with very high ceilings and changing out filters would be a major pain in the behind.
I see that in all the newer homes, it must be the new code here in Florida.