Headlight leak in '99 Miata

My 99 has always been garaged, until lately.  I live in San Diego
where it hardly rains and have never noticed moisture inside my
headlight lens, until now.  The car has been parked in the driveway
for several months, we've had some rain and now water has condensed
inside the driver-side lens; I can see no leak.  I've parked the car
in the sun****ne and popped the rubber seal off the inside to hopefully
let the water eva****ate, but to no avail.  Although I'm sure I can get
the water out, can someone tell me how this likely happened.  I have
no cracks in the lens and the seals look good.  Any ideas would be
welcomed.
-Rick