I have hit the passenger side mirror while driving through construction. One construction barrel was way into the lane, and my mirror clipped it just enough to scare me. Thankfully no damage.
The mirrors stick out on the 2012 Focus a LOT. With another car having mirrors the same, they could certainly make contact as the driver passes by..
The housing does rotate on a spring-like mechanism. So the housing could have taken a big hit and just moved, breaking the mirror with a hard enough hit, but leaving the cover intact. (and since the connection to the car is cracked, I bet it was a car driving by that did it.)
You should just buy a whole new mirror, and exclange the painted part. It is going to cost about $150. (as a guess.. I did one on my last Ford and it was $120. not painted)
((Save the old internal mirror adjustment motors.. They can be valuable if one dies later on..
The mirrors DO fold, and I suggest for folks who park on the street to fold the mirror when leaving the car overnight.(if they are worried)
I see a few folks who do this all the time. I guess they had a broken mirror too.