I too would love a feature added to the software to find the valves. There is a commercial product called Chat-R-Box you can find on Amazon.
I have lived in the same house for 24 years. The sprinkler system was already installed before I bought the house. And in 24 years, I have never found the 6th valve. The system recently developed a leak and I “lost” a different valve. I know the general area, but probing the area has not found it.
I set up two OpenSprinkler (Pi) systems back in 2015. They are still working great. I think the second house also has missing valves. I have not tried to locate them yet.