SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

I know this may seem an odd question, but any tricks to finding buried heads/sprinklers?
I have been working on re-starting a 30 plus year old dormant system on my property the past 2 years. just as i think i may have a zone up and going, I discover a wet spot and low and behold I unearth a bubbler/sprinkler. there are a couple area's left that dont have coverage, and in my mind I cant imagaine there is not a sprinkler head to cover that area.
So my question; Any tricks to finding these? Ive measured and mapped on a grid and found 2, dumb luck found 1.
this is an 8 zone system, with between 6 and 13 heads per zone.

not sure what good using air instead of water would do. I have all 8 zones working, just wonder how best to locate an unknown head/bubbler.