How to find a water leak?

So, I noticed water flowing out of the bulkhead wall at the rear corner of my property. That is not normal and typically does not happen even with heavy rains. I looked at my water meter and sure enough it was spinning away with no water demand in the house. I suspected a leaking sprinkler head. I shut off my water shutoff valve that turns off water to the house and the sprinkler system.  Water meter still spinning. Called city and guy came and closed shutoff at the meter. Leak stopped. How do I go about finding the leak? The only place I can find water is at the rear corner of my lot. Meter is located on same side front corner. Shutoff valve is in my garage centered on driveway so I assume water line goes from meter, along property line then does a 90 and heads under driveway to shutoff valve. A leak between meter and shutoff valve seems like it would show up in front yard since water tends to flow downhill. 

I suspect there is a union and some old like that runs down the side of my house and that is where the leak is. From what I can see so far I am dealing with PVC pipes. 

Any tips on how to locate leak? Time to start digging. Luckily it's Sunday so I can work on this without taking off work. 

Shovel.  Go to the softest part of the area and find the pipe.  Go to derpot and get a water main valve handle so you can turn it back on yourself to check for the leak once you find the pipe

I had that happen to me, but I found out about it by a water bill that was about 10x normal.  The line runs from the meter about 250 ft to my house and is down about 4 ft.  I called a plumber.  They have leak detection machines that may or may not work.  Theirs couldn't find it, so it was back to digging with the little digger bob cat thingie.  Found it.  It was 100' from the meter, or the length of one 100' plastic pipe section where it joined to the next section.  At the plumber's suggestion, I had them install a box down to the pipe and a valve there between the 2 sections of pipe, and another at the next 100' section junction.  Yeah, it wasn't cheap.

Based on where the leaking water made itself known, I don't see how the leak could be between the meter and the house shutoff. I suspect there is another water line, no longer in use, that runs beside the house to the back yard for who knows what reason. I did some digging where the water was flowing and just found wet soul. Ut no pipe. I then dug a hole next to the driveway where I suspected the water line was and found a 3/4" pvc pipe. The pipe that goes i to the shutoff in the middle of my driveway is a 1" pvc pipe, so that was odd. I dug a couple feet in each direction along the 3/4" pipe thinking there may be a tee and that I could cap off the pipe going to the backyard. Couldn't find a tee but confirmed that this pipe continues past the front of the house and goes along side it towards the back yard. I bought a wrench so I can turn my own water back on.  I just cut that 3/4" pipe and glued in a ball valve. After glue sets for a bit, I'll turn the water back on and hopefully have no more leak. 

In reply to Dr. Hess :

About once a year I give thanks that my water meter is inside the house.  All the older houses around me have a situation like yours and at least one a year has the same problem as you.  

After spending my whole day making my yard look a bit like a Caddyshack golf course by digging exploratory holes, I located a strange abandoned pipe that bypasses my shutoff valve and went to the backyard where it then discharged water somewhere in my backyard. I cut that pipe and installed a ball valve, then after testing it out, then capped it off. 

You all fixed up?  Maybe there were plans for an outbuilding when they ran the lines originally

Yeah, I'm all good now. No idea what the mystery water line went to, but I suspect at one time it may have been water to the dock. The water to my dock now comes from the house and has a shutoff valve in the garage. Just another mystery of buying a ore-owned house. Previous (and only) owner is named Furman, so I always refer to things like this as getting "Furmanized'. My hands are tired today since most of my digging was in the midst of a lot of roots and sprinkler lines that I did not want to destroy, so there was a lot of digging by hand - I don't mean by hand on a shovel, I mean clawing at the sand and pulling it out of my holes with my fingers.

Makes me want to dig up the line of view and wind blocking trees I have in that flower bed and just make the whole thing gravel so there won't be any roots if I have to dig there again.