I think you are going to have to "start cutting concrete".  If the leak is at the top of the hydrant, there is a seal.  If the leak is underground it is either the hydrant stem (which requires removal/replacement), hydrant-waterline connection (which requires digging/inspection/repair), or supply line leak (which may require even more digging to find and repair).  Not good news, but like said, better now than in 6 months!