I had a similar/ ?same problem.  My windshield like yours showed no signs of a
leak.  Yet i decided to try silicone sealer around all edges of the windsheild
anyhow.  Nipped it right in the butt, no more leaks!  I too live in the
(lately) rainy North East and have not felt a damp spot in that area since.
Took me about 30 min to do, and of course I choose a dry day :).
Chris
87 Syncro GL