ptg,

Some additional info needed. The controller is located where? How many zones are hooked up in the controller? Where is the mainline shut-off in relation to the controller?
You had a sprinkler contractor out to look at the system and all he could come up with is get a metal detector? If you can't find the valve how do you turn off the zone? Do you know where all the other auto valves are? Pictures would be helpful. Is the water source from the house(hose bib)or from the mainline to the house? These questions are a start. The forum is at a disadvantage because we can't see the your lot nor walk around it. The more info the more we can help. You may have to do some probing or listening. Aloha

A metal detector is unlikely to work because the valves are most likely to be plastic with very little metal in them. If you don't have any luck probing small low spots near the begining of the zone, it may require someone to do a wire trace for the valve.

Cut the line and put fish tape in it and follow with a metal detector.

Trace your pipes. My bet would be the valves are together, near your water line to the house. Also, look at the spray pattern. Your zones will have somewhere around 4 sprinklers, when the zone comes on, sprinklers will turn on in an order, the first in the zone will be a few seconds before the second.

Look at the parts of a sprinkler system to understand what you're looking for. And get another sprinkler guy.