I bought this 2011 CPO Flex last June, it came with a new battery. The positive terminal was pretty corroded when I finally got it home and looked the car over real close.
No big deal, I figured I'd clean it myself, that was 8 months ago.
Yesterday I finally did the cleaning job with a baking soda/water mix and various brushes and scrapers, it came out pretty clean, so far so good.
Next I installed those green/red anti-corrosion pads around the battery posts and gave everything a thin coating of dielectric grease.
I'd disconnected both terminals, when I went to reconnect them the horn alarm went off and the only way to stop it was to pull the negative terminal.
I tried this a few times but each time the horns alarm would honk. The Owners Manual was no help.
I was frustrated, I've cleaned lots of battery terminals on my cars with alarms and never had this happen.
I got on the internet and looked up this issue, a Mustang Forum had a suggestion and I tried it.
I put the key in the ignition, turned it to the Accessory position and then connected the negative cable, it worked, no alarm.
I learned something new.