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.