Today I had a similar event to the OP, multiple compass calibrations needed, however no catastrophe. I had flown the last three days, Fri, Sat and Sun with no problems near my home. I've never had to calibrate since I loaded 1.6, it's been working fine, as expected. Today I go to the beach (12mi east), didn't need to recalibrate, I flew two batteries, no problems. Third flight was a different story. Before launch everything looked great, MOD was around 1475 range, like the previous 2 flights. I got a green confirmation safe to fly, 12 satellites, all is good, battery was just charged 100%. So I take off and as I get about 150' away I have a RED warning of compass error. The craft automatically goes into ATTI mode, I could tell from the way it was flying. So I bring it back and carefully land it, fortunately there was no wind. This is the first time I've ever had a compass error mid-flight in the 6 months I've been flying this Phantom on previous firmware. So I do the compass dance and the craft LED lights confirm the calibration was completed successfully. However the RC still has the red compass error. Hmm. So I initiate another calibration and hold the drone a little higher, about 6', and do the dance. Again, the craft's LED lights confirm the calibration was completed successfully. And again, the RC
did not go green, safe to fly. Puzzled, I flicked the mode switch from P to ATTI, then back to P. That produces the green light safe to fly, so I thought I was good to go, but then it goes red with compass error again 10 seconds later. Then another 10 seconds later all by itself it goes back to green "safe to fly" and stays green. So I launch and hover for 30 seconds to make sure all is OK, and the flight went well as normal.
This is the ONLY time I've ever had to calibrate more than once, similar to the OP. The 1st calibration was 5' above blacktop, on an old road with no metal nearby anywhere. Keep in mind I had just launched from this same spot 45 min earlier, no problem. Pretty strange behavior, but it all worked out. The only reason I went through the time to report this is because these were two "firsts" for me. 1. Getting an error after launch. 2. Having to calibrate twice in what I would consider an open benign area for calibration. Oh yeah, my phone wasn't on me. I'm not blaming 1.6, I just find it curious because this has never happened before, then, a few days after loading 1.6 I get this surprise. Have any others had this occur all of a sudden with 1.6? I'm not sure if the OP had 1.6 or not, I don't think he disclosed that.
So, now I'm thinking that if a mid-flight compass error occurs like this, after landing, it's likely wise to turn everything off, and then back on to start a clean compass calibration. That's what I should have done, and was going to do, but when it stayed green after the second calibration for a minute, then continued green in the air, I figured it was good to go, same as the OP, but his story ends much different. Which makes me wonder, if the OP got a bad calibration from all the rebar nearby (that's what we think happened, right?), why didn't it give him a compass error after he left the parking structure?
Fortunately I completed my beach mission with no surprises. It was awesome today at San Onofre beach during the golden hour. Check this sunset.