I am uncertain of what methods they use to determine what substances are in the area. I only have the experience with the old military detectors, and they were notoriously unreliable. When they worked, they would alarm for nerve agent on some of the powdered fruit (particulates anyway) from the MREs. Reportedly, the technology used now is much better. Some of the systems in place at governmental locations (mail hubs, Senate buildings, the White House, etc) are also much better - all that is a long winded way of saying I am uncertain how they determined (initially) that it could be VX. It could be that the symptoms matched what is on a list somewhere and they acted according to protocols for that particular agent. So many of the symptoms for an allergic reaction, a "normal" reaction to certain reagents and reactions to nerve agents are similar, so there could have simply been some confusion and actions taken reflected the worst case scenario. Of course, as always, YMMV.