[Damian]

I had this same problem with my Thermarest recently. Only deflated when I used it. Very maddening. It wouldn't even deflate when I put a piece of plywood on it with a 110lb outboard motor on top and let it sit for days. Only when I used it. Submerging it in the hot tub yielded no leaks. I figured it was haunted and was about to discard it but then tried overinflating it, which you can do simply by blowing hard into it. The subsequent hot tub submersal test then indicated there was a threshold pressure above which it would leak significantly at a small spot that appeared to be delaminated. Once the pressure subsided a bit, the leak completely stopped. Strange behavior, but such is physics. Try overblowing it, and then look for the leak.

[Slugman]

Same thing for me, a haunted Thermarest pad. I just gave in and bought a new one, an Exped this time. I can’t really complain about the old pad, I got it used for cheap, a long time ago, and it already leaked at that time. But it leaks more now. I’m going to repurpose it into a swimming raft.