were the vents original equipment? if added, disclosure from the homeowner as to past problems is required and take you out of the picture.
More than likley they were added. Poor ventilation design found after a few years would dictate mechanical upgrades. Was the plywood delaminated? were the bathroom vents terminating in the attic?
Did you probe the plywood for delamination?
Did you walk the roof?
Aside from pics, all these things lead to the right answer to your question.