I'm having the exact same issue! So nice to hear there are others as well and I'm not alone on this. I have a 2018 focus st base package, and here is my issue currently, and here is what I have found so far.So I had the exact issue you describe happen to my 2015 Focus ST.
In the end (fingers crossed) it ended up being a hairline crack in the 50A fuse going to the BCM. This fuse is located in the BJB just in front of the positive terminal on the battery box. Yellow wire with a green stripe.
found it would make a circuit and then sometimes loose the circuit.
My mirror lights were flashing like yours. My car alarm would go off. My car would recognize my key fob etc.
What a pain.
Nice to hear you got an edge ST.
When I first bought the car used, I was driving it home and suddenly the throttle cuts, car sputters a little, and dies. I pull off to the side of the road with my head racing. Couple of strange things I noticed: 1. the front and rear right headlight and taillight were no longer on, 2. it was saying no key detected, but the battery in the fob was fine, and the secondary start method was not working either, telling me it's not the key, but the sensor in my car, and later findings showed it simply wasn't getting electricity.
I decide to wiggle the battery because yk, why not, I didnt want to have to pay several hundred to get it towed, and I had an interview for a job in the morning so I needed to get home and I was going to try everything I could.
I pulled on the main red battery terminal connector, and slid the battery forward, and i heard a slight beep like something finally connected. Now all the lights are on, and my key fob unlocks the doors and is being detected by my car. I hop in, and start driving, and after less then a minute, it shuts off again. I jump out of the car, pop the hood. pull the battery forward, hear the same beep, the lights are all on now and the key is now being detected. I drive the car several hours the rest of the way home.
Throughout the next couple weeks the issue continues, and I can re-create it upon hitting the acceleration which slides the battery back, killing the engine. I've checked all battery terminals, they're tight, and everything seems to work perfectly fine.
Recently, its been getting worse, and becoming more and more difficult to get started until last night, I can no longer get it to start. I decided to get precise and try to narrow down where the issue is occuring. I found after several hours of playing around with it that the plate in front of the battery where the main red battery connector goes to; if i move it less than a half inch the light comes on, but as soon as I release, they go back off and not key detected. I checked all connections and everything seems tight, and there is no corrosion. My mind is blown that I have it narrowed down to a specific small area, and still cannot find the issue, so my guess is the fuse like you are saying has a hairline crack. Did you ever get this solved?
I would absolutely love any feedback I can get from anyone, please dont tell me that it's a loose connection or a corroded battery terminal though, I've checked it all. It's something a little more precise.