I would be absolutely astonished if there are any SAMs, anywhere, that were at any state of readiness. I'd be equally astonished if even today there was anything in the ROE that permitted a launch against an airliner in a simple loss-of-contact scenario.
The exceptions, i.e. fortification around the Pentagon and MANPADS at the White House, have been in extremely unusual circumstances.
I would submit that surface to air missiles (SAMS), especially in the north central part of the country (i.e. Minneapolis), have never been present in any form, anywhere in that region.
There *might* have been some somewhere in that region at some time in the 50's or 60's when the Soviet bomber threat was at its highest, but a long time ago, for basic point defense purposes of assets such as capital cities or strategic locations (which in the Minnesota/Wisconsin/UP Michigan region would have been little to none over the years), it was decided, quite correctly in my view, that the defense of such areas lay in their location rather than from some surface-to-air missile defense. It would have been incredibly cost prohibitive to erect, man, maintain and justify any set of missile defense systems for a non-existent threat - especially one that has not been around for decades.
The A320, as well, flying at a typical flight level for a transit from KSAN to KMSP (probably FL33 or FL35), would have been out of an MANPAD (man-portable air defense, aka Stinger or SA-7 grail-type) or portable air defense weapon envelope by several thousands of feet.
As far as the Pentagon goes, prior to 9/11 there have never been any fortifications in or near or around the building - ever. The closest they ever came was the old NIKE/Hercules system that had 4 fixed sites, each between 15 to 25 miles away from the Pentagon area. These were de-commed in the early 70's. There is an old site in Lorton about 5 miles away where I live. These air defenses were designed to do what they could against Soviet incoming ICBMs. On the afternoon of 9/12, driving through the Pentagon parking lot, they had Avenger HUMVEE-mounted Stinger systems there. Since then, there are fixed Patriot (I believe) sites at 4 locations around DC. I've seen two of these - there may be more than 4, though.
The whole idea of fixed/planned air defense is to shoot something down way before it gets to a defended asset, hence these Patriot sites locates a few to 5 or more miles away from the strategic locations being defended. Point-defense systems like the Phalanx CIWS are true last-ditch defenses, where you might very well destroy the incoming threat but run the risk of having high-subsonic, supersonic or hypersonic missile debris still impacting your asset.
The myth of Pentagon SAM "standdown" is a key element of April Gallop's hilarious lawsuit (proudly supported by Cap't Bob and his merry minions from PfT) that renders, above all else in that POS suit, its certifiable death in the legal system.
Again, the location of our CONUS bases and facilities are the best defense - or *were*, prior to 9/11, against external air-breathing threats. Still, however, surface to air missiles should be and are used in extreme circumstances and are still extremely rare in our world.