Living in SW CT, I can tell you that your RH is too high, IMHO. Keeping
humidistat @ 25%, windows in some unheated rooms just start to show
condensation. (double-pane, low-e, of course.)

With that RH, small leaks into attic from room(s) below could easily
cause your problem. Poorly taped drywall joint, minor gaps here and
there, and the mold is off and growing. Previous owner of my house had
one small series of gaps in ceiling, maybe totaling 1 sq inch- provided
growth media for various green and black flora. (And ... for a list of
reasons, their impact on the inside RH was purely random.) Even on
inside of roof deck, you could see evidence of condensation, on most of
roof.

HTH,
J