Were the parking lots concrete? Concrete with rebar in it will do that.
I didn't believe it until I tried to launch from a family friends nice concrete front patio area. Got the "magnetic interference" thing for the first time. I asked her if there's rebar in this concrete and she said "yes". So I moved 5' off the concrete and launched from the grass with no issues! (I guess I could have hand launched if I'd thought of it...)
OK now PRM, I never had that problem until you mentioned it, probably cuz most of my flying was at home. But anyway, we were driving around today and ended up downtown, I had the the AB in the back and decided to get some video here, and sho-nuff, had the same problem until I found a different spot to lift off.