AC was the first place I called when my landlord let my horse out of his pasture (accident plus stupidity) and he was missing for 3 DAYS. Working with AC is how I got him back. People called in with sightings and then a horse person caught him and held onto him for a bit until he could be trailered back to my place. I would call AC and see if they have gotten any calls if someone is missing their horse odds are they would call AC eventually. Since most AC don't have facilities for horses they may just send someone out to check on her (mention her feet!) and see if you can hang onto her for a couple of days.
I hope you find her owner!

Pictures?