Question
I have a six zone system that was already installed when we bought our house.  The previous owner covered up the control valve boxes and so far I have been able to find two control valves which are just down line from the back flow valve.  (The valve box I found was covered with about 8 to 10 inches of bark mulch.)  Anyway, I have been poking around the rest of the flower beds trying to locate the other control valves.  (I assume the two I located split to 3 zones each and therefore that there are 6 move valves out there somewhere.)  So far, no luck.  Do you know of anything I can use to locate those other boxes?

Any suggestions would be appreciated.

Answer
Probably the most effective way would be to try to trace the wires from the controller out to the valves.  Some irrigation supply houses (John Deere Landscapes, Ewing, Horizon) have apparatus for that purpose.  I dont know if they can be rented or not.  There are also some irrigation service companies, or repair services that may have the equipment and would do the location for you.  Otherwise, it is just a matter of probing every square inch until you find something.