Where the rusty dust may appear
Fine reddish-brown spores from a dry rot fruiting body can settle on pipes, window ledges, cobwebs and flooring. The dust may be some distance from the fungal growth that produced it.
Rule out ordinary building dust
Brick dust, old plaster, rust and debris from building work can all look reddish or brown. The surrounding signs of decay and moisture need to support the identification.
Spores do not mean every nearby timber is infected
Spores are widespread. They need suitable moisture and temperature conditions before growth becomes established. Dry, sound timber should not be condemned merely because spore dust has settled on it.
Use spore dust as a reason to investigate
Where there is also fungal growth, dampness or timber cracking, the dust helps build the picture. The practical next step is to find the extent of affected timber and the cause of the dampness.
Useful technical sources
These links provide background guidance. A particular building still needs its own inspection.