It's 100% condensation. In order for the entire rig to be wet, you'd need a huge hole and it would be obvious.
When I had my rig under a TG365 cover in the Seattle area, I always kept two Goldenrod heaters under the cover. Goldenrod heaters were designed for use in gun safes to prevent condensation, and they work great under scope covers as well.
Another, much less obvious, head scratcher is that on a couple of occasions, I uncovered my rig to find the scope filled with water. The rig itself was dry, but there was enough water in the scope that I needed to dump it out. When it happened on my EdgeHD 8, I was able to dump the water out through the vents. When it happened with my C14 (without vents), I needed to remove the corrector plate to dump out the water.
It took me a while to figure out what was happening. Since it's such a rare occurrence, I'm still only 95% sure of the root cause. Anyway, in both cases, it happened after a fairly long stretch of freezing weather (and only twice in at least 5 years of having the rig out under cover full time). I think that the problem is that the TG365 cover was draped over, and in direct contact with, the scope. I think that this created a conduction mechanism where, even with the heaters, the heat inside the scope was able to escape through the cover. I think that this super-cooled the inside of the telescope and every bit of moisture under the cover condensed inside. To prevent the problem, I always fold a towel into many layers and drape that over the scope before putting on the cover. Since doing that, the problem has not recurred. Again though, since it's so rare, it's not 100% conclusive, although it's been many years (and winters) since it happened last.
In the last year, I've changed the way that I store my gear. I used to leave the rig out under cover and ready to go at any time. Because I do travel for astrophotography, I made some changes to my cabling and mounted much of the gear on top of the OTA, such that there are only two connections (one power and one USB) that need to be plugged or unplugged to mount or unmount the scope. And since all of the other gear is mounted on top of the scope, I add or remove it in one piece. It takes about 2 minutes to do the whole thing. Since it's so easy now, I only leave the OTA, camera, etc. on the mount when I know that I have clear nights in the immediate future. Otherwise, I just unmount the OTA and bring it all in, again in one piece.