Make a decision
Return leakage versus dust buildup
One scope
A leak upstream of the filter can pull unfiltered air from a cavity or basement.
The other scope
Accumulated dust is material already present in an accessible section.
A concrete example
A dusty return near a basement storage room may be drawing air through an open seam. Photograph the seam and ask whether sealing is in scope; vacuuming alone can leave the mechanism active.
Which question settles it?
Sealing and cleaning solve different mechanisms; ask which condition was actually observed.