Just ran into a very random problem and wanted to post the solution here in case anyone else runs across it. Hopefully it will save some frustration.
My wife and I had just driven the car earlier in the morning and there were no problems at all. With summer around the corned and winter hopefully past us, I decided I would put on the summer tires. I got in the car to position it better for changing tires and turned off the ignition. I could still here an engine running. I looked over my shoulder to see if someone had pulled in behind me, but there were no other cars to be seen. I then realized that it was my engine that was still running. Put the key in the ignition turned it to fully on and then off again nothing changed. The car continued at a running idle. I put on the parking brake and shifted to neutral, getting ready to pop the hood and take a poke around. Low and behold when I took my foot off the brake pedal the car's engine shut off. I replicated the conditions a couple of times, and the problem remained.
I set about changing my tires, as that was what I had pulled everything out to accomplish, and then started pondering about the cause of my phantom engine running problems. I had recently change the GEM module to add remote central lock to the car, and also poked at the wiring for a reverse light that wasn't working. I couldn't understand how these would cause this problem as we had driven the car since the work was done. Before I decided to start ripping apart my other work I decided to query the all knowing oracle, Google. I poured through a few Ford forums about other people having similar but different problems. Most of them revolved around fuel shut-off problems, but usually related to cars with the TDCI engines. Then there was one link I had skipped over on the front search page, a link to a Peugot forum. I had skipped it as I couldn't imagine a Peugot problem was related to my Ford in any way, although I had added Ford to my search queries. The main problem was about a poor Peugot design where the fuse boxes placed under the hood would collect water and cause problems. That was obviously irrelevant, but Google had not led me astray, in the last comment on that page there was a one line comment that was the solution to my problem.
"I had a similar problem with my Ford Fiesta turned out to be the two stage tail light bulb had burned and the high power broken filament had shorted to the other side." I figured I might as well check and after pulling apart the left tail light assembly I saw the exact problem on my Fusion. I popped the bulb out, started the engine, held my breath and turned the ignition key to off. The engine shut off instantly. Replaced the bulb and all is good again.
This is obviously a bug in the Ford electrical system, but easily rectified. Again I hope this helps someone else out.