Central Air main return is filtered but not the small returns from the rooms?

In my moms house, there is a large return grill at the top of the stairs with a 16x20 filter that feeds directly into a large return plenum. The filter is mounted just above the grill, i.e the grill holds the filter in place. Just above the filter is a large plenum that feeds to the furnace return.
Problem is all the individual return ducts from the various rooms all converge in this same large duct. Therefore the return air from the individual rooms does NOT pass through the filter. Only the return air from the main grill passes through the filter.
Am I missing something or was this how the installers built it?
Is the filter supposed to be installed somewhere up in the plenum and not right at the intake grill or ????
I looked for but did not find any other filter at the furnace or elsewhere in the system.
thanks
Mark
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If all return air passing through a filter it is Okay if not then somebody has screwed up
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responding to http://www.homeownershub.com/hvac/central-air-main-return-is-filtered-but-not-the-small-return-46890-.htm
stan wrote:
Mark wrote:

The filter should be installed to filter air from all the grills. Look in the furnace for a filter.

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