Thanks for the feedback guys. I am pretty sure this is condensation, since it hasn't gotten wet for a couple months (Sept), but the temperature has dropped from highs in the 70s to lows in the upper 20s The entire inside was damp, but no water pooled. Pretty sure it is not a leak. The cap is angled up when parked so water can't get past the threads when on the sidestand, and the small drain hole is facing down and well sealed with silicone. I doubt the cap is air tight, so the pressure differential idea may have merit. And it usually lives in a cool garage, so rolling it out into the sun on a warm day would cause the black plastic to warm quickly while still having cool air inside. Exhaust doesn't get hot enough to bother the saddlebag 3/4" away on the plastic heat shield, so I don't think it could affect the tube a foot away, especially with air flowing over the bike. If it is condensation, I can't be the only one. Mine is mounted in a fairly exposed location, but I see photos of them on the back of panniers all the time, so hopefully guys who have them mounted can comment.