Improper pluming vent through roof.

Did an inspection yesterday-house at least 100-yrs old and recently remodelled and upgraded, but without filed plans or permits. Three full baths and two kitchens in the same general area, so I expected a 4" vent stack through the flat roof. Go up to the roof-no vent in that area. Called it out to the owner,went to write my report, and realized from my photos that the cast-iron roof drain/strainer was in just the right spot for the vent. Do you suppose someone ran the drain right into the top of the vent stack? No way of telling for sure without major demolition(no attic access) so I just wrote it up as no visible roof vent. Funny thing is: all fixtures seemed to drain fine.