Roger Shoaf wrote:
....
At 70% humidity dew will not form on the ground, so it should not form in
your attic.

Not necessarily so...dew may well form on the ground just not in the air
as fog and may OP may well condense moisture out in the attic on cold
surfaces even if the bulk air temperature is above ambient. As some
other responder noted, protruding nails, etc., are prime candidates...

(Same phenomenon as the "sweating" of a water glass...