That line about cutting open a wall really got me, because a buddy of mine did the exact same thing last summer. He had water pooling by his furnace and swore up and down it was a slab leak, so he paid a guy $400 just to look at it. Turned out the hose on his humidifier had a pinhole in it, a $7 part from the hardware store. He patched the drywall himself and now he jokes that the hole is his reminder to check the cheap stuff first. The paper towel trick is smart, I'm going to remember that one.