Thanks. I went probing around today but came up empty. I did find the water valve for the line in from the street that had been covered over.
Here's a photo of the house from the street. The pipe comes out from the side of the house and then at some point goes under the driveway to the front yard. There's a telecom line that runs along near the sidewalk and that's making finding things a little more challenging.
I found the receipt from when the previous homeowner had the sprinkler system installed. It was 05/11/01. I emailed the installer asking if by chance they might have a diagram in their records that I could get a copy of. I use them when I winterize my system so maybe I'll get lucky. We'll see. But this blind poking in the ground isn't getting me anywhere.