How do I find a small leak on a down mat? By morning it's quite a bit deflated, but not all the way. The valve seems to remain tight...Any ideas? Is it ok to dunk it in water?

Inflate the pad so its reasonably firm. Dunk each section in the bath tub and watch for bubbles. I have tried several methods but this seems to work the best.

The bath tub method can work, but can also feel like trying to drown a large, buyont slug. The mat should be waterproof, so no worries there. Also, if the leak is really slow, it might not show a steady stream of bubbles.

My Exped Downmat 9 developed three slow leaks, and I was only able to find two using the bath tub. The last leak was undetectable.
However, if made by Exped, send it to OR and they will replace the mat. They have done this for MANY people as the older downmats almost always leaked.

If you have no success with the bath, I have been successful mixing in a little soap in with some water in a spray bottle, this will cause the bubbles to be much more noticeable on the pinprick punctures.

Good luck!

and some dish soap, with a paint brush do one section at a time (vertical or horizontal, whatever you can see, look for bubbles.... same trick used for puttin tires on wheeeeels...