I had one once that I couldn't see. I eventually just rolled a foot at a time in a puddle and found it pretty quick with bubbles coming up. Mine only leaked when the truck was parked with the weight on the smallest nail that had worked itself around the inner edge of the tread. Taking it off and using soap probably wouldn't have worked since it would usually hold air and I could go for weeks without ever parking on the exact spot that made it leak. Wood splinter trumps my story tho. Crazy.

 

how bout putting some fix a flat in it?

 

Fix a flat in terriable stuff. Highly corrosive to aluimnum. May damage TPMS sensors as well. Take it to a tire shop, they can dunk it in a tub a find the leak.