If the meter got moved for some reason, I could see somebody wondering what to do with the now-unused ground wire, and deciding that making it "look attached" was better than letting it hang free or cutting it off.Another idea - maybe the previous owner had some kind of antenna wire (TV, shortwave, ham radio) coming through this window, and this was a ground connection for that? 

Kevin, there is noting wrong with reservations and having your antenna up.  

Not deface, damage, impair, or remove any property, facilities, fixtures, wiring, equipment, and appliances; not install TV/radio antennas, decorations, not other equipment without landlord/agent approval; not exhibit any signs, advertisements, notices, or other lettering.

It gives the following blurb as examples of ineligible items:"Barbecue pits; bath houses; dumbwaiters; exterior hot tubs, saunas, spas, and whirlpool baths; outdoor fireplaces or hearths; photo murals; swimming pools; television antennas and satellite dishes; tennis courts; tree surgery. 

The awesomely wealthy Rockefeller once said that he knew it was time to sell his stocks in 1929 when his shoe shiner was offering stock tips.I see more and more normally non-RE folks talking about buying rentals so my froth antennae are up,  but I still see good demand for rentals and a tight supply of houses for sale in my area.  

We have an antenna for free tv.

Trust me, there's much worse, so keep your guard and antennae up.

Someone in a forum recently said (and it made my antenna stand straight up) that he could think of a million ways a property manager could steal his money.

I have Roku TVs and also provide an antenna for local stations.

Would an aerial antenna paired with good internet service and smart TVs be sufficient?