Yes, Plain City is on the Madison-Union County line just west of Columbus. I have a soft spot for it in my heart because it is a part of my childhood memories, full of Amish and Mennonite influence, and a nice place to live!
Tiny Unionville Center, just northwest of Plain City, also has an STL-10.
Also, Marysville recently demolished part of their downtown fire station, which had an STH-10 behind it. Thankfully, that siren has been saved and relocated to a spot on Raymond Rd on the west side of the city.
Just a quick question, but are any of those sirens still even there? The image dates are 2007 or 2009, and that was the time when Franklin began replacing their old sirens.
Here is a map I have put together of the Franklin County system circa 1974. It is still a work in progress. If you know of locations not listed, please feel free to post those locations.