lol Now can you imagine that happening to a horse crazed girl in her PJ's - just looking out the window and seeing a pony in the driveway hehe... when I was little I used to wake up every AM and run to the window to see if a pony would magically apear under the tree LOL Sounds dorkybut its the first thing that came to mind.
Anywhooo - I agree on calling AC - they'll be able to have it on record and if the owner calls they can direct them to you.
Signs are a good idea, as is posting on local newspapers that are online in the lost and found section etc...