Water leak under a slab?

I noticed a large "blister" of paint on the wall yesterday and when I popped it open the sheetrock underneath it was all wet. There is no water pipe in this wall but there is an ice maker line and an outside water hose connector that runs under this wall. The sill plate is SOAKED with water and I am assuming somehow that pipe under the wall must be leaking. I looked at the water meter and the little dial is not moving which would indicate no water is flowing so how do I find out if the pipe is leaking or if this is ground water from the lawn or what the heck it is coming from? It is under a concrete slab so I cant just look at it. It is located about 20 feet inside the house from an outside wall. There is no basement, therefore no sump pump. It has not rained here in 6 months and it is not running down the inside of the wall. I sponged it dry and 10 minutes later it is wet again. It is located about 20 feet inside the house from an outside wall. There is no basement, therefore no sump pump. It has not rained here in 6 months and it is not running down the inside of the wall. I sponged it dry and 10 minutes later it is wet again. It is located about 20 feet inside the house from an outside wall. There is no basement, therefore no sump pump. It has not rained here in 6 months and it is not running down the inside of the wall. I sponged it dry and 10 minutes later it is wet again.