Loosing 50 gallons of water per day

I manage a 40 unit condo property comprised of 3 buildings with one steam boiler heating all 3 buildings. We have a meter connected to the water feed to the boiler system to periodically check for undetected leaks. Recently we have noticed that the system is using approximately 40 -50 gallons of water per day. That's right per day!! We have thoroughly checked all areas of the 3 buildings looking for obvious signs of water but have not found anything. This now leads us to believe that somewhere in the 3 buildings under the concrete slab floor their is the possibility of a return line leak. We think that if there is a leak in an underground return line that the opening in the pipe is probably pointing towards the ground under the concrete slab allowing the water to be absorbed by the dirt and thus not being detectable anywhere else in the 3 buildings.
I am at a loss as to what to do. someone mentioned hiring a leak detection company. I have never used this type of service nor do I know any reputable company that performs this type of service in the Boston area.
Any suggestion's on locating the leak or finding a company that could do this would be greatly appreciated.

Thanks,
Larry